Systems and methods for controlling and forming polymer gels

ABSTRACT

In preferred embodiments, the present invention provides methods of controllably making a vinyl polymer hydrogel having desired physical properties without chemical cross links or radiation. The gelation process is modulated by controlling, for example, the temperature of a resultant vinyl polymer mixture having a gellant or using active ingredients provided in an inactive gellant complex. In accordance with a preferred embodiment, the method of manufacturing a vinyl polymer hydrogel includes the steps of providing a vinyl polymer solution comprising a vinyl polymer dissolved in a first solvent; heating the vinyl polymer solution to a temperature elevated above the melting point of the physical associations of the vinyl polymer, mixing the vinyl polymer solution with a gellant, wherein the resulting mixture has a higher Flory interaction parameter than the vinyl polymer solution; inducing gelation of the mixture of vinyl polymer solution and gellant; and controlling the gelation rate to form a viscoelastic solution, wherein workability is maintained for a predetermined period, thereby making a vinyl polymer hydrogel having the desired physical property. In further preferred embodiments, the present invention provides physically crosslinked hydrogels produced by controlled gelation of viscoelastic solution wherein workability is maintained for a predetermined period. In another aspect, the present invention provides kits for use in repairing intervertebral disks or articulated joints including components that form the vinyl polymer hydrogel and a dispenser.

CROSS REFERENCES TO RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 10/771,852 filed Feb. 2, 2004, which is a continuation-in-part ofU.S. patent application Ser. No. 10/631,491 filed Jul. 31, 2003, whichclaims the benefit of U.S. Provisional Application No. 60/400,899 filedAug. 2, 2002. The entire contents of the above-identified applicationsare hereby incorporated by reference.

BACKGROUND OF THE INVENTION

Lower back pain affects over 65 million people in the United States withan estimated 12 million of these cases arising from degenerative diskdisease. The back is particularly susceptible to damage and disease dueto its complex structure. The spine is a complex structure ofarticulated bone and cartilage comprised of a column of vertebraeseparated by vertebral disks (FIG. 1). These vertebral disks act as anintervening cushion to mitigate and distribute loads transferred alongthe spinal column.

The anisotropic structure of the intervertebral disk efficientlyachieves the appropriate mechanical properties required to cushioncomplex spinal loads. The inner viscoelastic material, termed thenucleus pulposus, occupies 20-40% of the total disk cross-sectionalarea. The nucleus usually contains between 70-90% water by weight. Thenucleus is composed of hydrophilic proteoglycans that attract water intothe nucleus and thus generate an osmotic swelling pressure rangingbetween 0.1-0.3 MPa, which supports the compressive load on the spine.The nucleus is constrained laterally by a highly structured outercollagen layer, termed the annulus fibrosus (FIG. 1). The nucleuspulposus is always in compression, while the annulus fibrosus is alwaysin tension. Although it comprises only one third of the total area ofthe disk cross-section, the nucleus supports 70% of the total loadexerted on the disk. The intervertebral disk becomes less elastic withage, reaching the elasticity of hard rubber in most middle-aged adultsas the nucleus loses water content. This water loss will also cause thedisk to shrink in size and will compromise its properties.

In degenerative disk disease, the nucleus pulposus can become distortedunder stress, resulting in the extrusion of part of the pulposus outthrough the annulus fibrosus, causing pressure against the surroundingnerves. This process is called herniation. The damage to the disk canoften be irreversible if part of the pulposus is lost. The majority ofdisk injuries occur in the lumbar region, and the most common area ofdisease occurs at L4/L5 and L5/S1.

A laminectomy (surgical removal of part of a herniated disk—typicallythe nucleus pulposus) may be performed to relieve pressure on localneural tissue. This approach is clearly a short-term solution, giventhat the load bearing ability of the nucleus would be reduced with lossof material. Despite this, over 200,000 laminectomies are performed eachyear, with a success rate of 70-80%.

Arthrodesis or fusion is a more permanent method for surgically treatingdegenerative disk disease. Fusion is accomplished with or withoutinternal fixation. While internal fixation has become increasinglypopular, this technique has its share of complications. Fracture,neurological damage, and osteoporosis have been observed in patients whohave undergone internal fixation fusions. The ability of the bone tofuse will vary from patient to patient, with the average likelihood ofsuccess ranging from 75-80%. Spinal fusion will cause stiffness anddecreased motion of the spine. Additionally, fusion can also put stresson adjacent vertebrae in the spine, which can accelerate disease inadjacent disks and lead to additional back surgery.

A successfully designed artificial disk would replace a worn out diskwhile protecting patients from incurring problems at an adjacent levelof the spine. Several artificial disk prostheses have been proposed inthe prior art. Many of these prosthesis attempt complete replacement ofthe disk, including the nucleus and the annulus fibrosus. Given that theintervertebral disk is a complex joint with multi-directional loading,the design of a prosthesis that mimics the articulation and mechanicalbehavior of a natural disk is extraordinarily difficult. For example,when the body is supine, compressive loading on the third lumbar disk is300 N, rising to 700 N in an upright stance, then to 1200 N when bendingforward by 20°. Additionally, moments of 6 N-m are often achieved duringflexion and extension, with up to 5° of rotation. For adequate safety, apreferred compressive strength of the entire disk is 4 MN/m².

The most extensive experience to date with a complete disk replacementis that obtained with the SB Charité III prosthesis. This prosthesis hasbeen used extensively in Europe since 1987, and has been implanted intoover 3,000 patients. The SB III is designed with a polyethylene spacerplaced between two cobalt chromium endplates. Two year follow-up studieshave shown good clinical success in patients. Another study concerned acomplete disk prosthesis consisting of a polyolefin core reinforced withcarbon black, which is attached to two titanium plates. Preliminaryresults are not promising, since the core fractured in 2 of theimplants.

Both of the examples presented above serve to indicate that there isconsiderable commercial effort being expended in the development ofartificial disks. However in both cases the mechanical equivalence ofthese components to the human intervertebral disks is somewhat doubtfuland the long-term clinical prognosis is still unclear.

As an alternative to the complete replacement of intervertebral disks,the nucleus pulposus alone can be replaced, leaving the annulus fibrosusintact. This approach is advantageous if the fibrosis is intact, in thatit is less invasive, and the annulus can be restored to its naturalfiber length and fiber tension. In replacing the nucleus, it isdesirable to find a material that is similar in properties to thenatural nucleus. The prior art describes bladders filled with air,saline, or a thixotropic gel. To prevent leakage, the membrane materialcomprising the bladder must be impermeable, which inhibits the naturaldiffusion of body fluid into the disk cavity, preventing access tonecessary nutrients.

To generate a more natural disk replacement material, several researchgroups have investigated polymeric hydrogels as a possible replacementfor the nucleus pulposus. Hydrogels are good analogs for the nucleuspulposus, in that they typically possess good viscoelastic propertiesand can offer good mechanical behavior. Additionally, they contain alarge amount of free water, which permits a prosthesis made from ahydrogel to creep under compression and handle the cyclical loadingwithout loss of elasticity, similar to a natural nucleus pulposus. Thewater permeability of these materials also allows diffusion of bodyfluid and nutrients into the disk space. Control of this pore structure,and the consequent control of the nutrient access to all parts of theimplant, may be critical for future prosthetic implants.

Others have investigated the use of polyacrylonitrile-polyacrylamidemultiblock copolymers encased in a jacket made from ultra high molecularweight polyethylene fibers. These systems absorb up to 80% of theirweight in water. Polyvinyl alcohol (PVA) and copolymers of PVA and polyvinyl pyrrolidone (PVP) have produced prostheses with mechanicalproperties similar to natural disks. These materials have the additionaladvantage of having clinical success in other medical devices. Gelsformed from PVA are usually prepared via a freeze-thaw process or viaexternal crosslinking agents. In addition, hydrogel-based nuclei cancontain therapeutic drugs which slowly diffuse out after implantation.Although no clinical data is currently available for these materials,biomechanical testing on cadaver joints has shown similar mechanicalproperties to natural disks

SUMMARY OF THE INVENTION

In preferred embodiments, the present invention provides methods ofcontrolling the gelation kinetics of vinyl polymers. These methodsinclude, in preferred embodiments, controllably making a vinyl polymerhydrogel having desired physical properties without chemical cross linksor radiation. The gelation process is modulated by controlling, forexample, the temperature of a resultant vinyl polymer mixture having agellant (also spelled as gellant) or using active ingredients providedin an inactive gellant complex.

Preferred embodiments include an injectable hydrogel such as forexample, for nucleus pulposus augmentation using minimally-invasivesurgical implantation of prosthetics fabricated in situ. The hydrogel inaccordance with preferred embodiments of the present invention conformto the region of interest, for example, vertebral surfaces in a jointspace. The load bearing hydrogels formed by in situ gelation methods ofthe present invention minimizes damage to, for example, the annul usfibrosus. In accordance with a preferred embodiment, the method ofmanufacturing a vinyl polymer hydrogel includes the steps of providing avinyl polymer solution comprising a vinyl polymer dissolved in a firstsolvent; heating the vinyl polymer solution to a temperature elevatedabove the melting point of the physical associations of the vinylpolymer, mixing the vinyl polymer solution with a gellant, wherein theresulting mixture has a higher Flory interaction parameter than thevinyl polymer solution; inducing gelation of the mixture of vinylpolymer solution and gellant; and controlling the gelation rate to forma viscoelastic solution, wherein workability is maintained for apredetermined period, thereby making a vinyl polymer hydrogel having thedesired physical property. In further preferred embodiments, the presentinvention provides physically crosslinked hydrogels produced bycontrolled gelation of viscoelastic solution wherein workability ismaintained for a predetermined period. In another aspect, the presentinvention provides kits for use in repairing intervertebral disks orarticulated joints including components that form the vinyl polymerhydrogel and a dispenser.

In certain preferred embodiments, the step of providing a vinyl polymersolution typically includes the step of dissolving the vinyl polymer inthe first solvent. The step of mixing the vinyl polymer solution with agellant may precede or follow the step of heating the vinyl polymersolution to a temperature elevated above the melting point of physicalassociations of the vinyl polymer.

The desired physical property typically includes at least one of lighttransmission gravimetric swell ratio, shear modulus, load modulus, lossmodulus, storage modulus, dynamic modulus, compressive modulus,cross-linking and pore size. In preferred embodiments, the desiredphysical property is physical cross-linking.

In preferred embodiments, the vinyl polymer is selected from the groupconsisting of polyvinyl alcohol, polyvinyl acetate, polyvinylpyrrolidone and mixtures thereof. Preferably the vinyl polymer is highlyhydrolyzed polyvinyl alcohol of about 50 kg/mol to about 300 kg/motmolecular weight. In preferred embodiments, the vinyl polymer is highlyhydrolyzed polyvinyl alcohol of about 100 kg/mol molecular weight.Typically the vinyl polymer solution is about 1 weight percent to about50 weight percent solution of polyvinyl alcohol based on the weight ofthe solution. In preferred embodiments, the vinyl polymer solution isabout 10 weight percent to about 20 weight percent solution of polyvinylalcohol based on the weight of the solution.

In certain preferred embodiments, the method includes the step offurther contacting the viscoelastic solution with a gellant, typicallyto modify the physical or chemical properties of the resulting gel. Thismethod is suitable for producing a local modification in the physicalproperties of the gel, such as to maintain the gel in place within abody cavity, such as a space within an intervertebral disk.

In preferred embodiments, the first solvent is selected from the groupconsisting of deionized water, dimethyl sulfoxide, an aqueous solutionof a C₁ to C₆ alcohol and mixtures thereof. Preferably the gellant ismore soluble than the vinyl polymer. In certain preferred embodiments,the vinyl polymer is introduced into an aqueous solution of a gellant.

Typically, the Flory interaction parameter of the mixture of vinylpolymer solution and gellant ranges from 0.25 to 1.0. In preferredembodiments, the Flory interaction parameter of the mixture is at least0.5, more preferably about 0.25 to about 0.5.

Typically the gellant is selected from the group consisting of salts,alcohols, polyols, amino acids, sugars, proteins, polysaccharides,aqueous solutions thereof, and mixtures thereof. In preferredembodiments, the gellant is selected from the group consisting ofchondroitin sulfate, dermatan sulfate, hyaluronic acid, heparin sulfateand mixtures thereof. In other preferred embodiments, the gellant isselected from the group consisting of biglycan, syndecan, keratocan,decorin, aggrecan and mixtures thereof. In further preferredembodiments, the gellant is an alkali metal salt, most preferably sodiumchloride.

The gellant may be added as a dry solid or in solution. For example,solid NaCl can be added to an aqueous solution of vinyl polymer, oradded as an aqueous solution of sodium chloride from about 1.5 molar toabout 6.0 molar, more preferably about 2.0 molar to about 6.0 molar. Infurther preferred embodiments, the gellant is an aqueous solution of analcohol chosen from the groups consisting of methanol, ethanol,i-propanol, t-propanol, t-butanol and mixtures thereof.

The gellant may be in an active form or an inactive form when it ismixed with the vinyl polymer solution. In preferred embodiments, thestep of inducing gelation of the viscoelastic solution includes the stepof activating the gellant. Preferably, the inactive gellant is activatedby a controllable trigger event.

In certain preferred embodiments, the inactive gellant is amacromolecule and the active gellant comprises fragments of amacromolecule that are released by cleavage of the macromolecule. Insome embodiments, the cleavage of the macromolecule is enzymaticcleavage; a preferred macromolecule is a physiological substrate of theselected enzyme. In preferred embodiments, the macromolecule is selectedfrom the group consisting of chondroitin sulfate, dermatan sulfate,keratan sulfate, hyaluronic acid, heparin, heparin sulfate and mixturesthereof and the enzyme is selected from the group consisting ofchondroitinase ABC, chondroitinase AC, chondroitinase B, testicularhyaluronidase, hyaluron lyase, heparinase I/III and mixtures thereof. Inother preferred embodiments, the macromolecule is selected from thegroup consisting of biglycan, syndecan, keratocan, decorin, aggrecan,perlecan, fibromodulin, versican, neurocan, brevican and mixturesthereof and the enzyme is selected from the group including, forexample, without limitation, aggrecanase and mixtures thereof.

In other embodiments, macromolecule can be thermally denaturable; insuch embodiments, a preferred macromolecule is collagen. Alternatively,cleavage of the macromolecule is by irradiation with electromagneticradiation or particulate radiation.

In further preferred embodiments, the inactive gellant is a bad solventsequestered in a vesicle, a liposome, a micelle or a gel particle. Insome preferred embodiments, the liposome is a phototriggerablediplasmalogen liposome. In alternate preferred embodiments, the liposomeundergoes a phase transition at about the body temperature of a mammal.Preferably, the liposome includes, without limitations a mixture ofdipalmitoylphosphatidylcholine and dimyristoylphosphatidylcholine.

In further preferred embodiments, the inactive gellant is associatedwith a gel particle that is in an active form up on undergoing a phasetransition at about the body temperature of a mammal. In suchembodiments, the gel particle suitably comprises a polymer selected fromthe group consisting of poly(N-isopropyl acrylamide-co-acrylic acid),N-isopropylacrylamide, hyaluronic acid, pluronic and mixtures thereof.In other preferred embodiments, the gel particle releases its contentsupon undergoing degradation.

Typically, the rate of gelation is controlled to provided an adequateperiod of workability needed for further processing of the viscoelasticsolution, including injecting, molding or calendaring. In preferredembodiments, the viscoelastic solution is injected into an actual orpotential space in the body of a mammal. In particularly preferredembodiments, the viscoelastic solution is injected into anintervertebral disk or an articulated joint, such as a hip or knee. Thehydrogel can be retained within the body space by virtue of generalphysical properties or by its local physical properties at the injectionsite. In preferred embodiments, desired local physical properties can beadjusted by a further addition of gellant near the injection site. Inother preferred to embodiments, the hydrogel can be retained within thebody space by the use of known medical devices to seal, reinforce orclose the injection site or other defect of the body cavity. Suitablesuch devices are disclosed in published International PatentApplications WO 01/12107 and WO 02/054978, which are hereby incorporatedby reference in their entirety.

In other preferred embodiments, the step of processing includes coveringa burn or a wound.

The preferred embodiments of the present invention provide methods ofmaking a gel and controlling a property of the gel. In accordance with apreferred embodiment of the present inventions a method for making a gelincludes comprising dissolving a vinyl polymer in a first solvent toform a vinyl polymer solution and introducing the vinyl polymer solutionin a volume of a second solvent to cause gelation, the second solventhaving a higher Flory interaction parameter at a process temperaturethan the first solvent. The Flory interaction parameter (χ) isdimensionless and depends on, for example, temperature, concentrationand pressure. Solvents can be characterized as having a low χ value orsolvents having a higher χ value wherein χ=0 corresponds to a solventwhich is similar to a monomer. A solvent having a higher χ value ischaracterized as a solvent that causes a gelation process at atemperature. A thetagel, in accordance with the present invention, isformed by using a second solvent having a Flory interaction parameterthat is sufficient to cause gelation.

Preferably the second solvent used in the preferred embodiment has aFlory interaction parameter in the range of 0.25 to 1.0. Typically,first and second solvent characterstics are chosen to allow use of themethod of the preferred embodiment at room temperature or at bodytemperature of a mammal. The gel produced by the method of the inventionhas physical cross-linking, and is substantially free of chemicalcrosslinking agents. In a preferred embodiment, the vinyl polymer ispolyvinyl alcohol.

In some embodiments, a plurality of cycles of contacting the vinyl in animmersion solvent (second solvent) and contacting with the first solventare performed. Alternatively, the method may include subjecting the gelto at least one freeze-thaw cycle. The polyvinyl alcohol (PVA) hydrogelsthus may be both a thetagel and a cryogel. Partial gelling can beaccomplished with either method and then completed using the other, oreven alternating between the two methods.

While the examples and discussion herein are directed towards vinylpolymers and in particular PVA hydrogels, thetagels can be made in asimilar manner using any polymer that possesses the appropriate kind ofphase diagram as described hereinafter with respect to the Floryinteraction parameter. A mechanical force can be applied to the gelduring the gelling process, changing the manner in which it gels andalternatively producing oriented gelation.

In several embodiments, the vinyl polymer is highly hydrolyzed polyvinylalcohol of about 50 kg/mol to about 300 kg/mol molecular weight. Inother embodiments, the vinyl polymer is highly hydrolyzed polyvinylalcohol of about 100 kg/mol molecular weight. The vinyl polymer solutionis about 1 weight percent to about 50 weight percent solution ofpolyvinyl alcohol based on the weight of the solution. Preferably, thevinyl polymer solution is about 10 weight percent to about 20 weightpercent solution of polyvinyl alcohol based on the weight of thesolution.

The first solvent is selected from the group of solvents having a low χvalue that is not sufficient to enable gelation, i.e., solvents in whichthe energy of interaction between a polymer element and a solventmolecule adjacent to it exceeds the mean of the energies of interactionbetween the polymer-polymer and the solvent-solvent pairs, as discussedbelow. In several embodiments, the first solvent is selected, withoutlimitation, from the group consisting of deionized water, dimethylsulfoxide, an aqueous solution of a C₁ to C₆ alcohol and mixturesthereof.

In general, the immersion solution comprises a solvent having a high orsufficient χ value that enables gelation. In some preferred embodiments,the immersion solution is an aqueous solution of a salt of an alkalimetal, typically sodium chloride. In other embodiments, the immersionsolution is an aqueous solution of a C₁ to C₆ alcohol, typically anaqueous solution of an alcohol chosen from the groups consisting ofmethanol, ethanol, i-propanol, t-propanol, t-butanol and mixturesthereof. In certain embodiments, the immersion solution is an aqueoussolution of methanol.

In general, the vinyl polymer gels of the present invention can be madein-situ for applications such as filters, microfluidic devices or drugrelease structures in situations in which freeze-thaw gelation may bedifficult or impossible to execute.

In one embodiment, the vinyl polymer solution is placed in a chamberhaving at least two sides and a membrane. The membrane has propertiesthat contain the vinyl polymer while providing access to small moleculesand solvents.

In some embodiments, the vinyl polymer solution is separated by themembrane from at least two different immersion solvents, typically afirst immersion solvent and a second immersion solvent. In someembodiments, the first immersion solvent is an aqueous solution ofsodium chloride from about 1.5 molar to about 6.0 molar. In someembodiments, the second immersion solvent is an aqueous solution ofsodium chloride from about 2.0 molar to about 6.0 molar. In otherembodiments, the first immersion solvent is a 1.5 molar aqueous solutionof sodium chloride and the second immersion solvent is an aqueoussolution of sodium chloride from about 2.0 molar to about 6.0 molar. Insuch embodiments, a gradient in chemical potential is formed across thevinyl polymer solution between at least two different immersionsolvents. In one embodiment, a gradient in chemical potential is formedacross the vinyl polymer solution of about 4 mol·cm⁻¹.

In general, a gradient of a property is formed across the vinyl polymergel that corresponds to the gradient in chemical potential formed acrossthe vinyl polymer solution. Typically, the property is at least one oflight transmission, swell ratio, shear modulus, load modulus, lossmodulus, storage modulus, dynamic modulus, compressive modulus,cross-linking and pore size.

In some embodiments, one or both immersion solvents are changed in atemporal pattern to modulate the spatial gradient of a physicalproperty. Such temporal cycling is done on a time scale shorter than thediffusion time to make an inhomogeneous gel. In this way, gels can beproduced with a similar set of properties on the edges or peripheralregion and another set of properties in the central region, such asgreater cross-linking in the peripheral region as compared with thecentral region. Temporal cycling of immersion solvents can also be usedto modify the structure of the gel, for example, pore size, forproduction of filters. In such filters, small, locally varying pore sizemay be useful for some forms of chromatography (through size exclusion)or any other filtering application that requires pore size control.

Additional compounds can be combined in the physically cross-linked gel,including but not limited to, ionic or non-ionic species such ashyaluronic acid, polyacrylic acid and therapeutic agents.

In one embodiment, the invention provides a physically cross-linkedhydrogel comprising at least about 10 weight percent poly(vinyl alcohol)solution gelled by immersion in about 2 to about 3 molar sodium chloridewherein the hydrogel is about 14 percent to about 21 percent physicallycross-linked. In such an embodiment the final gel comprises about 12 toabout 29 percent poly(vinyl alcohol).

The preferred embodiments of the present invention also provide articlesof manufacture comprising a vinyl polymer gel having at least onegradient of mechanical properties, PVA thetagels may be used as abiocompatible load bearing or non-load bearing material for replacement,repair or enhancement of tissue. In general, PVA thetagels can replacePVA cryogels in applications where PVA cryogels are used.

In one embodiment, a one-piece prosthetic intervertebral disk is madecomprising a polyvinyl polymer hydrogel wherein the distribution ofmechanical properties of the one-piece prosthetic intervertebral diskapproximates the spatial distribution of the mechanical properties ofthe combination of the nucleus pulposus and the annulus fibrosis of thenatural intervertebral disk.

High compression PVA thetagels can be made by placing PVA in a reverseosmosis membrane with NaCl and then making the outside concentration ofNaCl quite high to compress PVA/NaCl. The NaCl concentration will climbas water leaves the reverse osmosis membrane gelling the PVA at highpressure. The concentration of PVA can be modified by the ratio of NaClto PVA inside the reverse osmosis membrane.

In a preferred embodiment, gel microparticles can be made throughgelation during agitation or by dropping blobs of fluid into acrosslinking solvent, such as the immersion solution.

In a preferred embodiment, gels can be embedded with particles thatdegrade (or do not adsorb) to “imprint” a pattern (“empty spaces”) onthe gel or as the drug release centers. Embedding neutrally chargedpolymers of varying molecular weights can be used to “space fill” thegel. These polymers are removable after the process, leaving acontrolled pore structure. Materials that are sensitive to freeze-thawcycles can be encapsulated. The gels can be embedded with particles orpolymers that are electrostatically charged to provide extra repulsionat high compressions but are collapsed in high salt. Such embeddedparticles can be those that are active in some manner (e.g. forcatalyses). Hydroxyapatite particles or other osteoinductive particles,agents, and similar moieties can be embedded to encourage bony ingrowthfor possible cartilage replacement.

In a preferred embodiment, poly(vinyl alcohol) gels can be used tocontain and release bio-active compounds such as growth factors,fibronectin, collagen, vinculin, chemokines and cartilage includingtherapeutic agents. The teachings with respect to incorporation oftherapeutic agents of U.S. Pat. Nos. 5,260,066 and 5,288,503 are hereinincorporated in their entirety. Contained compounds such as therapeuticagents or drugs can be released over time to modulate the local growl ofnormal tissues such as bone, blood vessels and nerves or tumors.

Temporal modulation of immersion solvents can produce thetagels inaccordance with the preferred embodiment with appropriate structure andphysical properties for containing and releasing drugs and otherbioactive molecules. In one embodiment, an outer skin is formed that ishighly crosslinked and an inner layer containing the drug/active agentis only weakly crosslinked. In such an embodiment, the outer skin is therate limiting component and has a constant release rate. Thus, drugrelease in accordance with a preferred embodiment includes the releaseprofile that is tunable by controlling the spatial gradient in PVAcrosslinking.

In one embodiment, the present invention provides a method ofcontrollably modulating the mechanical properties and structure ofhydrogels. In a preferred embodiment, the present invention providesarticles of manufacture with one or more gradients of mechanicalproperties that more closely match the existing gradients of suchproperties in natural structures. In one embodiment the inventionprovides prosthetic hydrogel articles of manufacture that mimic themechanical behavior of natural structures. In a preferred embodiment,the invention provides polyvinyl alcohol prosthetic intervertebral disksthat mimic gradients of mechanical properties found in the naturalintervertebral disks. In another preferred embodiment, the inventionprovides a one-piece prosthetic intervertebral disk that mimics thespatial distribution of the mechanical properties of the nucleuspulposus plus annulus fibrosis of the natural intervertebral disk.

In a preferred embodiment, particulates may also be added to the gel. Asdescribed hereinbefore, particulates can be added to create a controlledpore structure. Further, in accordance with another preferredembodiment, particulates can be added to provide a particularnanostructured gel. The particles can be either charged or uncharged andallow PVA crystals to nucleate at the surface of the particles.Particles that can be added include, but are not limited to, inorganicor organic colloidal species such as, for example, silica, clay,hydroxyapatite, titanium dioxide or polyhedral oligomeric silsesquioxane(POSS).

In accordance with another preferred embodiment, particles are added toprovide a charge effect to change the compressive modulus of the gel,and preferably increase the compressive modulus. This embodiment can usea thetagel having added particles. Upon compressing the gel in a saltsolution, a structure having particles with close packing whileshielding their charges results. Upon rehydrating with, for example,deionized (DI) water, the charge fields expand and results in a gel intension. This allows the gel to approximate physical properties ofcartilage, for example, at high charged particulate loads.

In accordance with another preferred embodiment, particulates are addedto the gel structure to provide mechanical properties such as, forexample, wear resistance. The addition of hardened glass (silica) ordifferent clays can provide wear resistance to the gels.

In accordance with another preferred embodiment of the presentinvention, a method for making a gel and controlling a property of thegel includes forming a thetagel as described hereinbefore by using afirst solvent to form a vinyl polymer solution and subsequentlyintroducing a volume of a second solvent to cause gelation, followed bypromoting dehydration to controllably structure the gel. This methodresults in uniformly structuring the gel and homogenizing the physicalcrosslinking of the PVA thetagel. This structure can be achieved byimmersing the contained PVA solution into a solvent which has a Floryinteraction parameter that is higher than the theta point for the PVAsolvent pair, and subsequently immersing the contained PVA in anothersolvent having a Flory interaction parameter lower than the theta pointfor the PVA solvent pair. The process can continue with successivedecreases in the Flory interaction parameter until the desiredinteraction parameter value for the gel is reached.

In another method in accordance with a preferred embodiment of thepresent invention, the PVA solution can be subjected to a graduallychanging solvent quality through a similar range of electrolyteconcentrations by the gradual addition of a concentrated NaCl solutionto deionized water such that the change of the salt concentration isslower, or equal, to the diffusion process of the gel.

In accordance with another preferred embodiment of the presentinvention, the PVA solution may be subjected to at least one freeze-thawcycle to fix the gel into a particular shape and then be immersed in aseries of solutions with successively higher Flory interactionparameters until the final desired Flory parameter is reached.Alternatively, the PVA solution is subjected to the one or morefreeze-thaw cycles after being immersed in a solution of 2 M NaCl.

In one preferred embodiment, nanoparticles are dispersed into solutionsof PVA. The solvent may be water, dimethyl sulfoxide (DMSO), methanol orany other solution that exhibits a Flory interaction parameter that islower than the theta point for the PVA solvent pair during solutionpreparation. The PVA/nanoparticle mixture is then subjected to at leastone freeze-thaw cycle. Subsequent to the freeze-thaw cycling, the gelledPVA is immersed in a solvent that has a Flory interaction parameter nearor higher than the theta point for the PVA/solvent pair to inducefurther physical crosslinking of the PVA/nanoparticle mixture.

Another aspect of the embodiments of the present invention furtherprovide methods of controlling the rate of gelation of polymer gels bychanging in the manner in which the polymer molecules interact. Bycontrolling the rate of gelation, a “window” of time that allows therelatively slowly gelling PVA solution to be manipulated or worked, forexample, by injection, molding or any other processing step that may bedependent on the flow of the gelling PVA solution. In preferredembodiments, the gelling PVA solution is injected into a region ofinterest such as a body cavity and gels in situ to form a PVA product.Preferred body cavities include the nucleus pulposus and a normal orpathological void within a joint.

In preferred embodiments, the rate of gelation can be controlled byholding the polymer, preferably PVA, above its crystallizationtemperature, thus preventing gelation even if the solvent quality ispoor. In other preferred embodiments, the rate of gelation can becontrolled by using a second solvent that can be triggered to changefrom good to bad. In some preferred embodiments, the quality of thesolvent can be changed by disruption of micelles. In other preferredembodiments, the quality of the solvent can be changed scission of highmolecular weight molecules. In further preferred embodiments, a poorsolvent is used in combination with process temperatures that acceleratethe gelation process.

The foregoing and other features and advantages of the systems andmethods for controlling and forming polymer gels will be apparent fromthe following more particular description of preferred embodiments ofthe system and method as illustrated in the accompanying drawings inwhich like reference characters refer to the same parts throughout thedifferent views. The drawings are not necessarily to scale, emphasisinstead being placed upon illustrating the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are a diagrammatic representation of spinal anatomyshowing transverse process, spinous process and vertebral body of thevertebral bones, the spinal cord and spinal nerves, and the nucleuspulposus of the intervertebral disks. The annulus fibrosis of theintervertebral disk surrounds the nucleus pulposus on lateral, anteriorand posterior sides.

FIG. 2 is a graphical representation of the relationship of the Floryinteraction parameter, χ, to the concentration (φ) of a polymer at agiven temperature in accordance with preferred embodiments of thepresent invention. The abscissa, at χ=0.25, separates polymer solutionsin first solvents (below) and polymer solutions in second solvents(above). The arrows and diamonds indicate the effect on χ produced byreplacing solvent 1 with solvent 2.

FIG. 3 shows 10% PVA solution in dialyzer cassettes after 1 day (top)and 3 days (bottom) of immersion in curing solution in accordance with apreferred embodiment of the present invention. From left to right: 1.5 MNaCl, 2.0 M NaCl and 3.0 M NaCl. The 1.5 M solution does not gel thePVA, the 2.0 M solution and 3.0 M solution do gel the PVA. Note theprogressive opacification of the 2.0 M gel and the shrinkage of the 3 Mgel from the edges of the cassette as the sample compacts with time(indicated with arrow).

FIG. 4 shows a uniform thetagel in accordance with a preferredembodiment of the present invention. PVA gels generated by immersion in3.0 M (left image of each pair) and 2.0 M (right image) NaCl curingsolution. Note that the gels are uniform and opaque. The gel exposed to3.0 M NaCl swells less and is more compact following equilibration indeionized water.

FIG. 5 shows an example of a gradient gel in accordance with a preferredembodiment of the present invention using a 10% PVA solution exposed tospatially varying NaCl concentration. Note the variation in both thetranslucency of the gel and in the swelling ratio.

FIG. 6 shows a differential scanning calorimetry (DSC) thermogramcomparing the results obtained with a thermally cycled PVA cryogel andthe “thetagel” in accordance with a preferred embodiment of the presentinvention. The solid line is indicative of 10% PVA immersed in 3.0 MNaCl for 3 days; the dashed line is indicative of 10% PVA thermallycycled 4 times from 10 degrees Celsius to −20 degrees Celsius with awarming rate of 0.02 degrees Celsius/min.

FIG. 7 graphically illustrates the relationship between the percentageof PVA in PVA hydrogels that were fully equilibrated in deionized waterafter being gelled in immersion solutions of different molarities inaccordance with a preferred embodiment of the present invention. Theconnected points represent measurements of 10% PVA immersed for 3 days,the single point represents an initial solution of 20% PVA solutionimmersed in 3 M NaCl for 12 days. For the 20% PVA solution, the 3 dayvalue of swelling ratio and percentage of PVA matched that of the 10%PVA solution (not shown). After 12 days of immersion in 3 M NaCl (and 5days of equilibration in deionized water), the 20% PVA solution formed agel that was 29% PVA.

FIG. 8 graphically illustrates the gravimetric swelling ratio for PVAhydrogels that were fully equilibrated in deionized water after beinggelled in immersion solutions of different molarities in accordance witha preferred embodiment of the present invention. The connected pointsrepresent measurements of 10% PVA immersed for 3 days, the single pointrepresents an initial solution of 20% PVA solution immersed in 3 M NaClfor 12 days. For the 20% PVA solution, the 3 day value of swelling ratioand percentage of PVA matched that of the 10% PVA solution (not shown).

FIG. 9 shows the dynamic modulus of PVA thetagel at 3 M NaCl, 20%initial PVA concentration and 1 N static load versus aging time in daysin accordance with a preferred embodiment of the present invention.

FIG. 10 shows the complex modulus (Storage (G′) and Loss (G″) Modulus)of PVA thetagel at 20% initial PVA concentration and 0.25 N static loadagainst solution molarity (2M and 3M NaCl) in accordance with apreferred embodiment of the present invention.

FIG. 11 is a schematic diagram of an “Ussing” type chamber used tocreate a gradient gel 160 in accordance with a preferred embodiment ofthe present invention.

FIG. 12 illustrates a quick freeze deep etch (QFDE) image of PVA gelstructure in accordance with a preferred embodiment of the presentinvention wherein the PVA gel is formed by immersion in 5 M NaCl for 3days. The bar represents 100 nm.

FIGS. 13A and 13B are a cross-sectional and a close-up view of thecross-section of a PVA gradient hydrogel, respectively, prepared byfilling Plexiglass tubing with 10% PVA solution, performing one freezethaw cycle (8 hours at −21° C.; 4 hours at room temperature) thenimmersing in 3 M NaCl bath for at least 3 days, then dehydrating in airfor 60 hours and returning to deionized (DI) water in accordance with apreferred embodiment of the present invention.

FIG. 14 illustrates a cross-sectional view of a PVA gradient hydrogelprepared by filling dialysis cartridge with 10% PVA solution, thenimmersing in a chamber having 3 M NaCl on one side and 6 M NaCl on theother side for 3 days in accordance with a preferred embodiment of thepresent invention.

FIG. 15 illustrates a close-up view of the cross-section of the PVAgradient hydrogel of FIG. 14 on the 6 M NaCl side prepared by fillingthe dialysis cartridge with 10% PVA solution, and then immersing in achamber with 3 M NaCl on one side and 6 M NaCl on the other for 3 days.

FIG. 16 illustrates a nanostructured PVA hydrogel prepared by mixing asolution of 10% PVA and 2 weight percent Laponite clay, subjecting to a1 freeze-thaw cycle, then exposing to a 4 M NaCl solution for at least 3days in accordance with a preferred embodiment of the present invention.

FIG. 17 illustrates a nanostructured PVA hydrogel prepared by mixing asolution of 10% PVA and 4 weight percent of silica, titrating to pH=3,subjecting to a 1 freeze-thaw cycle, then exposing to a 4 M NaClsolution for at least 3 days in accordance with a preferred embodimentof the present invention.

FIG. 18 illustrates a nanostructured PVA hydrogel prepared by mixing asolution of 10% PVA and 4 weight percent silica, titrating to pH=10,subjecting to 1 freeze-thaw cycle, then exposing to 4 M NaCl solutionfor at least 3 days in accordance with a preferred embodiment of thepresent invention.

FIG. 19 illustrates a nanostructured PVA hydrogel prepared by mixing asolution of 10% PVA and an octatetramethylammonium polyhedral oligomericsilsesquioxane (OctaTMA POSS) in water, then subjecting to 1 freeze-thawcycle in accordance with a preferred embodiment of the presentinvention.

FIG. 20 graphically illustrates the storage modulus for hybrid andcontrol PVA gels in accordance with a preferred embodiment of thepresent invention.

FIG. 21A illustrates a flow chart of a method of forming a PVA hydrogelin accordance with a preferred embodiment of the present invention.

FIG. 21B illustrates a flow chart of methods of forming and providing avinyl polymer hydrogel in accordance with preferred embodiments of thepresent invention.

FIGS. 22A-22D illustrate a PVA hydrogel prepared by adding 1.4 g of 400molecular weight poly(ethylene glycol) (PEG 400, Sigma Aldrich) to 6 gof an aqueous 10 wt % PVA solution while mixing, in accordance with apreferred embodiment of the present invention, showing the product atfour time durations after the end of mixing. FIG. 22A, zero minutes;FIG. 22B, 15 minutes, FIG. 22C, 2 hours, under a mineral oil protectivelayer; and FIG. 22D, one day, out of a jar.

FIGS. 23A-23E illustrate a PVA hydrogel prepared by adding 35.6 g ofaqueous 10 wt % PVA solution to 18.7 g of aqueous 5.1 M NaCl whilemixing, and the resulting mixture aggressively shaken, in accordancewith a preferred embodiment of the present invention, showing theproduct at five time durations after pouring into a covered dish and aflexible bag: FIG. 23A, zero minutes; FIG. 23B, 20 minutes; FIG. 23C, 1hour; FIG. 23D, 2 hours; and FIG. 23E, 17 hours.

FIGS. 24A-24F illustrate a PVA hydrogel prepared by adding NaCl toaqueous 10 wt % PVA solution at about 95 degrees Celsius (FIG. 24A)while mixing to make a final concentration of 2M NaCl (FIG. 24B), inaccordance with a preferred embodiment of the present invention. Aftermixing for 15 minutes, aliquots of the resulting mixture were pouredinto two containers that were cooled at room temperature (FIG. 24C, 15minutes; FIG. 24E, one hour) or on shaved ice (FIG. 24D, 15 minutes;FIG. 24F, one hour).

FIGS. 25A-25F illustrate the PVA hydrogels of FIGS. 24A-24F afterstorage. FIG. 25A, cooled one hour at room temperature and stored 12hours at room temperature; FIG. 25B, cooled one hour on ice and stored12 hours at room temperature; FIG. 25C, cooled one hour at roomtemperature and stored one month at room temperature; FIG. 25D, cooledone hour on ice and stored one month at room temperature; FIG. 25E, thePVA gel of FIG. 25C, oriented to show water released due to syneresis;and FIG. 25F, the PVA gel of FIG. 25D, oriented to show water releaseddue to syneresis.

FIGS. 26A-26D illustrate a PVA hydrogel prepared by adding NaCl toaqueous 10 wt % PVA solution at 95 degrees Celsius while mixing to makea final concentration of 2.1M NaCl, in accordance with a preferredembodiment of the present invention, wherein FIG. 26A shows a moldformed by a chilled polyethylene liner and the matching ball from atotal hip replacement joint, FIG. 26B shows the mold after filling thechilled polyethylene liner with the PVA solution and putting thematching ball in place, FIG. 26C shows the molded PVA in thepolyethylene liner after one hour in air at room temperature followed byone hour in deionized water at room temperature; and FIG. 26D shows themolded PVA product removed from the polyethylene liner.

FIGS. 27A-27B illustrate PVA hydrogels incorporating chondroitin sulfate(CS) prepared in accordance with preferred embodiments of the presentinvention, where FIG. 27A shows a PVA hydrogel formed by adding warm CSsolution at about 80 degrees Celsius to a aqueous 10 wt % PVA solutionat about 60 degrees Celsius to produce a 5 wt % PVA, 7 wt % CS mixtureand FIG. 27B shows a PVA hydrogel formed by adding 600 mg CS directly to10 ml aqueous 10 wt % PVA solution.

FIGS. 28A-28C illustrate flow charts of methods for forming a PVAhydrogel in accordance with preferred embodiments of the presentinvention.

FIGS. 29A-29F schematically illustrate a method for forming anddispensing a vinyl polymer hydrogel in accordance with a preferredembodiment of the present invention.

FIGS. 30A-30F schematically illustrate a method for forming anddispensing a vinyl polymer hydrogel in accordance with an alternatepreferred embodiment of the present invention.

FIGS. 31A-31E schematically illustrate a method for forming anddispensing a vinyl polymer hydrogel in accordance with an alternatepreferred embodiment of the present invention.

FIG. 32 schematically illustrates a dispenser for providing a vinylpolymer hydrogel in accordance with a preferred embodiment of thepresent invention.

FIG. 33 illustrates a midsagittal cross-section of a portion of afunctional spine unit in which two vertebrae and the intervertebral diskare visible.

FIG. 34 illustrates a transverse section through a damagedintervertebral disk, showing a spinous process and transverse process ofan adjacent vertebra, annulus fibrosis and, nucleus pulposus.

FIG. 35 illustrates a step of dispensing a mixture of gellant and vinylpolymer in a method for repairing a damaged intervertebral disk inaccordance with a preferred embodiment of the present invention.

FIG. 36 illustrates a step in a method for repairing a damagedintervertebral disk in accordance with a preferred embodiment of thepresent invention showing, in particular, a mixture of gellant and vinylpolymer, a sealant, a fixative, and the fixative delivery instrument.

FIG. 37 illustrates a step in a method for repairing a damagedintervertebral disk in accordance with a preferred embodiment of thepresent invention showing, in particular, a mixture of gellant and vinylpolymer, barrier, a fixative, and a fixative delivery instrument.

FIG. 38 illustrates a step in a method for repairing a damagedintervertebral disk in accordance with a preferred embodiment of thepresent invention showing, in particular, the mixture of gellant andvinyl polymer substantially filling the space previously occupied by thenucleus pulposus, a barrier, and a dispensing tube.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The preferred embodiments of the present invention are directed at thegeneration of uniform PVA hydrogels without chemical crosslinks orirradiation to produce a biocompatible material suitable for use as, forexample, an intervertebral disk prosthesis. Further, a preferredembodiment of the present invention includes a method used to create thePVA gels that result in a new class of PVA hydrogels which can bedesigned for specific applications to have a potentially large range ofmechanical properties, being controllable, and which can be engineeredwith gradients in structure and physical properties.

As used herein, “theta solvent” refers to a solvent that yields, at thetheta temperature, solutions of a polymer in the theta state. Thetasolvents may be composed of a single solvent or mixture of two solvents,a mixture of a solvent and a nonsolvent, or even a mixture of twononsolvents in the case of co-solvency as described by Elias, H. G.,“Theta Solvents,” in Brandrup, J, and E. H. Immergut, Polymer Handbook3^(rd) Ed., John Wiley and Sons, New York, 1989, the teachings of whichare herein incorporated by reference in their entirety.

As used herein, “gelation” refers to the formation of permanent physicalcrosslinks due to the crystallization of the PVA. This is in contrast tosome of the literature in which gelation refers to the point at whichthe polymer associates, and need not be followed by the crystallizationthat forms a permanent link.

As used herein, “associate” refers to a thermodynamically, or solvent,driven process whereby the polymer (or other species) “prefers” to be inan environment similar to itself, rather than in close proximity toanother species. This association of the polymer locally thennecessarily leads to a “phase separation” that may or may not lead to aninhomogeneous solution.

While not being held to a particular theory, it is thought that forcingpoly(vinyl alcohol) polymer chains in solution into close proximityusing a theta solvent through a spinodal decomposition mechanism resultsin the formation of a physical association that is resistant todissolution. As used herein, spinodal decomposition refers to aclustering reaction in a homogeneous, supersaturated solution (solid orliquid) that is unstable against infinitesimal fluctuations in densityor composition. The solution therefore separates spontaneously into twophases, starting with small fluctuations and proceeding with a decreasein the Gibbs energy without a nucleation barrier.

The methodology used in the present invention generates a PVA hydrogelemploying the controlled use of solvents having a χ value sufficient tocause gelation to force the PVA chains to physically associate. Toprevent random “crashing out” of the PVA, it is critical that thesolvent quality is controlled carefully, and, in particular for largercomponents, that the solvent “front” enters the PVA solution in acontrolled manner. For example, NaCl/deionized (DI) water andmethanol/deionized water solutions at temperatures and concentrations inthe neighborhood of their “theta” value for PVA were used to force thephysical association and subsequent gel ling of the PVA. Gels formed inthis way are called “thetagels” herein.

In general, by controlling solvent quality one can create a “window” oftime that allows the relatively slowly gelling PVA solution to bemanipulated or worked. This manipulation includes, without limitation,injection or molding or any other conceivable processing step.

The methods are applicable to the creation of materials for use inmedical, biological and industrial areas including the controlleddelivery of agents (which may include proteins, peptides,polysaccharides, genes, DNA, antisense to DNA, ribozymes, hormones,growth factors, a wide range of drugs, imaging agents for CAT, SPECT,x-ray, fluoroscopy, PET, MRI and; ultrasound), generation of loadbearing implants for hip, spine, knee, elbow, shoulder, wrist, hand,ankle, foot and jaw, generation of a variety of other medical implantsand devices (which may include active bandages, trans-epithelial drugdelivery devices, sponges, anti-adhesion materials, artificial vitreoushumor, contact lens, breast implants, stents and artificial cartilagethat is not load bearing (i.e., ear and nose)), any application wheregradients (single or multiple) in mechanical properties or structure arerequired.

The mechanism of gel formation includes a phase separation usuallyconsidered to be a spinodal decomposition process followed by acrystallization mediated by hydrogen bonding in the PVA rich regions ofthe solution. The choice of the solvent is known to influence thefreeze-thaw process. It is also known that some solvents can be used togel PVA without dropping the temperature of the solution below thefreezing point of the solvent. Solvents such as DMSO, ethylene glycol(EG), N-methyl-2-pyrrolidone (NMP), and gelatin have been used to gelPVA in ambient temperatures. The preferred embodiments of the presentinvention include the use of salt as a means of gelling a PVA solution,and the utility of manipulating and controlling the solvent qualityduring the gelation process.

An ideal polymer chain in a good solvent has no association behaviorthat leads to precipitation. As the solvent quality drops below thetheta condition, the chains begin to prefer to associate untileventually the polymer phase separates. In an ideal polymer, there areno further interaction forces in the polymer chains, other than thatprovided through thermodynamic (solvent) forces, and the normal,reversible phase diagram thus applies. However, in PVA, there is astrong binding force driven by hydrogen bonding that results in a localcrystallization of the chain. This crystallization can occur even whenthe solvent is “good.” Although the chemical potential in a good solventdoes not encourage association, random thermal motions do allow thechains to briefly contact and occasionally overcome this “solvationforce”. At this point, if the conditions are correct, as is the case forPVA, the chains will start to crystallize. This rate of initial contactto initiate crystallization depends on the solvent quality. Therefore,as the solvent quality is decreased, the probability of the PVA chainassociating increases since this is driven by the affinity of the chainfor itself, which is described by the solvent quality. As a result, rateof change of solvent quality is important, as has been observed bymanipulation of temperature. Since this process is related to thesolvent quality, it depends on temperature, concentration and pressure.Therefore, the gelling of the PVA must depend on time, and moreimportantly, a competition between the association “time” and the phaseseparation “time”. This competition has been observed experimentally,resulting in different initial get structures depending on cooling ratesand solvents, and in the aging of PVA gels.

Therefore controlling solvent quality by controlled solvent addition, orby temperature, pressure or any other relevant parameter allows one tocontrol the rate of this association. Consequently, one desires tocontrol the solvent quality of the system such that it is poor enough toaccelerate the association rate by promoting the proximity of the chainswhile ensuring that the solvent is not so bad that the polymer falls outof solution before crystallization can occur. The optimal conditions forgel formation are likely to be in the range of χ=0.25 to 0.5, but evenoutside these values it is possible to produce gels. By balancing thesecompeting effects, a polymer/solvent solution can be obtained that isstill fluid but gels rapidly.

For a single solvent with its solvent quality varied by temperature, orpressure, control, the spinodal decomposition phase transition isaccelerated by the decrease in solvent quality. For a ternary blend, thephase separation can be understood as either the PVA is poorly solvatedby the (continuous) water/solvent B blend and thus phase separates underthe identical mechanism as the single solvent system; or both the PVAand solvent B are solvated by water, but have varying miscibility witheach other. These materials therefore behave like a binary blend and,therefore, phase separate. Although conceptually slightly different, theresults are almost identical. The behavior of two good solvents thatresult in a poor solvent is known as co-nonsolvency. This approachprovides the ability to mix the components externally and then injectthe PVA solution into a region of interest such as a cavity wherein itsubsequently gels. This ability to inject PVA and induce gelation insitu is especially suitable for nucleus replacement or augmentation ofintervertebral disks (IVD), since the second solvent can be anythingthat has the required thermodynamic properties. This second solvent canbe a long chain polymer, protein or other complex molecule, or anaqueous solution thereof, such as polyethylene oxide (PEO), aglycosaminoglycan (GAG) or gelatin or simpler molecules such as simplyionic species like salts, for example, NaCl or short chain moleculeslike poly(ethylene glycol), for example PEG 400, glycerin or amino acidslike serine or glycine. Although higher molecular weight species are notcommonly characterized as solvents, the underlying theory, as describedby Flory, makes no distinction as to the length of the solvent molecule,although the final interaction parameter for the solvent/polymer pair isaffected by the solvent size.

PVA is known to associate at room temperature in deionized (DI) waterover long times. This is well below the theta transition of PVA in purewater of 97° C. The crystallites generated in both the freeze-thawprocess, and the thetagel process as used herein have a melt peak near60° C. In addition, most studies involving PVA require elevatedtemperatures of at least 80° C. to ensure that the PVA is fully“dissolved”. These observations suggest that above this temperature thePVA no longer possesses the ability to crystallize, and hence behaveslike an ideal polymer with respect to solvent quality. Consequently, itis recognized herein that if one can dissolve the PVA in a solutionabove this critical melt temperature there can be no crystallization,and hence no gelation, although there may be precipitation, or phaseseparation, due to simple association. If the solvent used is poor thenthere is some association through thermodynamic effects (and thusinhomogeneous precipitation may occur), but there can be no irreversiblegelation that characterizes the PVA hydrogels. However, as soon as thetemperature is lowered, crystallization can occur and the system canbegin to gel. This gelation crystallization is still however ratelimited, thus allowing the solution to be workable, even whilst belowthis temperature, and in a poor solvent. Mixing may influence thisgelation rate by encouraging collision of the polymer chains.

In general, a physically cross-linked poly(vinyl alcohol) gel isprepared from an aqueous poly(vinyl alcohol) solution (from 1% to 50%PVA by weight of the solution) that is gelled by contacting with asolvent having a χ value sufficient for gelation, hereinafter called thesecond solvent at a concentration approximating the “theta”concentration for the poly(vinyl alcohol) solution.

The present invention provides methods of producing poly(vinyl alcohol)hydrogels that do not use chemical cross-linkers, irradiation or thermalcycling. In preferred embodiments, the solvent quality is controlled,preferably by controlling the diffusion of the second solvent (NaCl ormethanol) into a PVA solution to produce a homogenous, physicallycrosslinked structure.

The present method uses a controlled change in solvents differing insolvent quality, conveniently expressed by the Flory interactionparameter to force the PVA to associate. Because no chemicalcross-linkers are used, the gel is substantially free of chemicalcrosslinkers and thus likely to be as biocompatible as thermally-cycledPVA cryogels. Any residue of NaCl in the gel following equilibration indeionized water is likely to be benign, as its concentration willcertainly be below physiologically relevant values.

FIG. 2 illustrates the relationship between the first solvent and thesecond solvent in terms of the Flory interaction parameter, χ. FIG. 2 isa graphical representation of the relationship of the Flory interactionparameter χ, to the concentration (φ) of a polymer at a giventemperature. The abscissa, at χ=0.25, separates polymer solutions infirst solvents (below) and polymer solutions in second solvents (above)sufficient to cause gelation. The arrows and diamonds indicate theeffect on χ produced by replacing solvent 1 with solvent 2.

By implication any solvent can be “good” or “bad” as defined using theFlory interaction parameter, χ. In terms of χ, these solvency propertiesare roughly χ<0.5 for good solvents and χ>0.5 for bad solvents, with thecondition of χ=0.5 defining a “theta” solvent. The good-bad solventtransition is not a step change, but is instead a gradual variation inthe solubility of the polymer in the chosen solvent. Consequently,changing the solvent quality changes the affinity of the polymer withitself, either through intra-chain interactions if the solution issufficiently dilute, or through inter-chain interactions. The effects ofchanging solvent quality on polymer solubility can be seen inexperimental data, or theoretically as shown in equation 1:v=(1−2χ)a ^(d)  (1)where □ is excluded volume of a single chain, χ is the Flory interactionparameter and a^(d) is the monomer volume. The theta point, where thechain is unperturbed, is therefore where v=0 or χ0.5. For v<0, phaseseparation occurs and there is an equilibrium between nearly puresolvent and a polymer-rich phase. However, due to the large size of thepolymer molecules discussed herein, there is always some polymer in thesolvent phase, and similarly there is always an appreciable amount ofsolvent in the polymer rich phase. The solvent can be a single chemicalspecies, or a mixture of species that is not necessarily limited to lowmolecular weight compounds.

It is usually considered that PVA phase separates through a spinodaldecomposition process. Note that there is no rate dependent effect inthis equation, although the spinodal decomposition process does have acharacteristic rate, and it is a balance of rate effects that isexploited in the embodiments of the present invention. For PVA in water,the Flory interaction parameter is weakly dependent on concentration but□>0.5 for polymer volume fractions greater than about 5%. The PVA gelhas a higher interaction parameter than the solvent but there is nodependence of χ on molecular weight or crosslink density of the gel.

In preferred embodiments, χ of the second solvent must be more positivethan the χ of the first solvent (dissolved PVA solvent) and ispreferably be in the range of 0.25 to 2.0. Preferably χ of the firstsolvent is in the range of 0.0 to 0.5. In general, the temperatureduring processing may vary from just above the freezing point of the PVAsolution to the melting point of the physical crosslinks formed in theprocess. The preferable range is from about 0 degrees Celsius to about40 degrees Celsius. Note that χ is coupled to temperature andconcentration. In preferred embodiments the temporal and spatial changein χ of the PVA solution (the first solvent) is controlled by contactwith another miscible solution (comprising the second solvent), whereinthe second solvent modifies the first.

The removal of the need for chemical crosslinkers and radiationprocessing allows a greater variety of embedded components. For instancemany bioactive materials are highly intolerant of chemical crosslinkersand radiation. In addition although in general the freeze-thaw processis gentle on bioactive components there can certainly be envisagedpolymers, biopolymers or cells that either cannot be frozen, or act asantifreeze hence preventing the freezing.

The method used to create these thetagels is likely to produce a widerrange of material properties and greater control over the physicalstructure of the final gel than is possible with competing cryogels. Theadvantage of thetagels over thermally cycled PVA gels for certainapplications is outlined below.

Cryogels have a fairly low resolution with regard to their finalproperties because each thermal cycle produces a dramatic change in thematerial properties of the gel. The thetagels produced demonstrate thatthe concentration of the solvent produces a monotonic decrease inswelling ratio once the theta value is passed (See FIG. 7). It isdesirable that the ultimate crosslink density is adjustable inproportion to the resolution achievable in the solvent concentration.For example, for the 10% PVA solution immersed in NaCl the weightpercentage of PVA in the final gel varies at a rate of about 7%/moleNaCl.

The preferred embodiments of the present invention provide gels having astarting weight (based on the weight of the solution) from about 1weight % to about 50 weight % PVA. In preferred embodiments, the weightpercent ranges from about 12% to about 29% PVA in the final gel wherethe immersion solution used was about 2.0 about 3.0 M NaCl (aq). Thisrange of final PVA weight percentages is comparable to that which can beachieved by thermally cycled PVA. With higher NaCl solutions (in excessof 6.0 M) percent PVA of the final gel increases monotonically with NaClconcentration. It has been found that PVA thetagels can be made thatexhibit a smooth gradient in spatial properties. In contrast, gradientsof properties cannot easily be manufactured in cryogels. Instead, theusual approach is to generate an array of stacked lamellae independentlythat must be joined in dissolved PVA and then cycled again. Sharpdifferences in modulus in such an array would create a material withundesirable mechanical properties and with inhomogeneous interfaces. Inpreferred embodiments, PVA thetagels having a smooth gradient inmechanical properties, can be used to make a prosthetic intervertebraldisk a central lower modulus “pulposus” having adequate compressivestrength and a higher modulus peripheral “annulus” that minimizes creepand undesirable distortion.

Modulus enhancement can be accomplished by incorporation of ionicspecies. For thetagels produced in NaCl, it is possible to includenatural (hyaluronic acid) or synthetic (PAA) polymers to create gelswith strain variable compressive moduli. Gelling a PVA/PAA solution instrong NaCl shield the ionizable charges in the PAA while the PVA iscrosslinked around the collapsed PAA. Re-equilibration in deionizedwater will allow expansion of the PAA and pre-stress the PVA matrix. Theresulting construct should have a very different mechanical compressivemodulus due to the repulsion of the fixed charges on the incorporatedPAA.

It is known in the art that PVA elicits little or no host biologicalresponse when implanted in animals. For this reason, PVAs are used in avariety of biomedical applications including drug delivery, cellencapsulation, artificial tears, artificial vitreous humor, contactlenses, and more recently as nerve cuffs. However, PVA has generally notbeen considered for use as a load bearing biomaterial primarily becauseof its low modulus and poor wear characteristics. The loads that anyvertebral implant must withstand will be reasonably high (on the orderof 4 MPa in compression) requiring a high compressive modulus. In vivo,the compressive axial load on the intervertebral disk is transferred bythe nucleus pulposus to a tensile circumferential load in the annulusfibrosis. Any biomaterial intended to replace the function of anintervertebral disk in its entirety must incorporate similar anisotropicproperties.

To improve overall strength, PVA modulus and wear characteristics can beenhanced by the formation of either chemical or physical cross-links.Cross-linking PVA by the addition of chemical agents (such aspolyaldehydes), through irradiation, or by freeze-thaw cycling, has beenshown to improve the durability of PVA gels. However, chemical additivescan leave unwanted residual reactive species behind that make the finalproduct unsuitable for transplant, while irradiation may adverselyaffect any bioactive material encapsulated in the matrix. Thus, thegeneration of extensive physical cross-links through freeze-thaw cyclinghas substantially improved the durability of PVA without the negativeside effects produced by chemical or irradiation induced crosslinking.Recent investigations suggest that the physical crosslinks produced byfreeze-thaw cycling might generate biomaterials with moduli suitable foruse as biocompatible replacements for load bearing structures such asarticular cartilage or intervertebral disk.

Solvation of Polymers and the “Theta” Point

Polymers in solution are complex molecules in perpetual dynamic motion.The configuration of an ideal polymer chain is usually described as a“random walk”, where the molecule is assumed for simplicity to be freelyjointed and free to move where it will. This behavior results in thepolymer assuming a spherical shape with a Gaussian distribution. Inreality the chain has a number of forces acting on it to define itsshape and behavior. In free solution the chain is subject to randommotion from Brownian fluctuations arising out of the temperature of thesystem. At the same time there is a force arising out of how the chaininteracts with itself (since it is a long, extended molecule) and itssurroundings.

If the polymer is easily solvated by the solution (i.e., it is in afirst solvent not having a χ value sufficient for gelation) it swells asit tries to maximize the amount of polymer chain that is exposed to thesolvent. In the first solvent, the energy of interaction between apolymer element and a solvent molecule adjacent to it exceeds the meanof the energies of interaction between the polymer-polymer andsolvent-solvent pairs as described by Flory, P. J. in, Principles ofPolymer Chemistry, page 424, Cornell University Press, 1953, theteaching of which are herein incorporated by reference in theirentirety. The chain is now in a perturbed state and resists contact withneighboring chains and equally resists mechanical compression anddeformation. As the solvency changes, this swollen configurationcollapses as the quality of the solvent falls.

At the theta point, the solvent quality is such that the random Brownianmotions are enough to keep the chain in an ideal, Gaussian distribution.Below this critical threshold the chain segments prefer to be next toeach rather than to a solvent molecule, and the chain shrinks (i.e. asecond solvent having a χ value sufficient for gelation). The Floryinteraction parameter, χ is dimensionless, and depends on temperature,pressure etc. The first solvents have a low χ, while the second solventshave a high χ, with a transition at about χ=0.5. The case χ=0corresponds to a solvent which is very similar to a monomer. In alattice model this is the case where the free energy comes entirely fromthe entropy associated with various chain patterns on the lattice. Insuch a case, temperature has no effect on structure, and the solvent issaid to be “athermal.” Athermal solvents are a particularly simpleexample of good solvents. In most cases the parameter χ is positive asdescribed by de Gennes, P. G. in, Scaling Concepts in Polymer Physics,First ed. p. 72: Cornell University Press (1979). If the solvent qualityis poor enough, the chain will completely precipitate out of solution.This effect can also be obtained by manipulation of the temperature ofthe solution.

Once the concentration of the polymer solution is high enough,adjustment of the solvent quality can be achieved by replacing at leastpart of a first solvent with a second solvent that forces inter-chaininteraction as well as intra-chain interaction. Once the physicalcrosslinking has occurred, the later presence of a good solvent, whichnaturally swells the free polymer, is balanced by the physicalcrosslinking. With inter-chain associations the polymer chains are nowconstrained at certain pinning-points. Consequently as the polymer issolvated, and stretches, it becomes more deformed and is forced intotension. It is the competition between the solvation of the polymerchains and this tension in the deformed chains that give gels theirinteresting mechanical behaviors. In addition, under certain conditionsthe polymer chains can be ionized, consequently generating a charge.Adjacent like charges will result in further swelling due toelectrostatic repulsion. This is part of the mechanism that givesnatural cartilage (collagen and glycosaminoglycans) its high modulus,and high hygroscopic properties.

Gelation Mechanism in PVA

Freeze-thaw cycling of solutions of PVA polymer results in the formationof physical cross-links (i.e. weak bonding through an “association” ofthe polymer chains). PVA hydrogels formed in this manner are termed“cryogels” and are described, for example, in U.S. Pat. Nos. 6,231,605and 6,268,405, the teachings of which are incorporated herein byreference in their entirety. Importantly, the techniques utilized tocreate PVA cryogels do not require the introduction of chemicalcrosslinking agents or radiation. Cryogels are therefore easily producedwith low impact on incorporated bioactive molecules. However,incorporated molecules are limited to those that can tolerate thefreeze-thaw cycles required to make the gel. Thus the resulting materialcan contain bioactive components that will function separately followingimplantation. PVA cryogels are also highly biocompatible (as will be theproposed PVA “thetagels” to be presented later). They exhibit very lowtoxicity (at least partially due to their low surface energy), containfew impurities and their water content can be made commensurate totissue at 80 to 90 wt %.

There is still some debate over the exact mechanism that drives thegelation of PVA through a freeze-thaw cycle. However, three models havebeen proposed to explain the physical crosslinking that occurs duringthe freeze-thaw cycle: 1) direct hydrogen bonding; 2) direct crystalliteformation; and 3) liquid-liquid phase separation followed by a gelationmechanism. The first two steps suggest that the gel forms through anucleation and growth (NG) phase separation, whereas the third optionpictures the process as a spinodal decomposition (SD) phase separation.Hydrogen bonding will form nodes and crystallite formation will formlarger polymer crystals. However both of these mechanisms will formclosely connected crosslinks, with relatively small crosslinking nodes.This observation is supported by studies on the gelation mechanism ofPVA. Spinodal decomposition on the other hand causes redistribution ofthe polymer into polymer rich and polymer poor regions followed by agelation process which results in more distantly spaced crosslinks. Itis thought that phase separation through spinodal decomposition islikely to be responsible for the improved mechanical properties of PVAafter crosslinking and occurs due to a quenching of the polymersolution. During the freezing process, the system undergoes a spinodaldecomposition whereby polymer rich and poor phases appear spontaneouslyin the homogeneous solution. This process occurs because the phasediagram of quenched PVA (and polymers in general) at certaintemperatures can have two coexisting concentration phases. The polymerrich phases are therefore highly concentrated which enhances the natural(weak) gelation of the PVA.

For cryogels, the physical characteristics depend on the molecularweight of the uncrosslinked polymer, the concentration of the aqueoussolution, temperature and time of freezing and the number of freeze-thawcycles. Thus the properties of a cryogel can be modulated. However,since the material's properties change dramatically at every freeze-thawstep, control over the properties of the finished gel is somewhatlimited. The thetagels described broaden the range of functionalitycurrently provided by PVA cryogels.

In general, the modulus of the PVA cryogel increases with the number offreeze-thaw cycles. In one experimental series, thermally cycled PVAcryogels had compressive moduli in the range of 1-18 MPa and shearmoduli in the range of 0.1-0.4 MPa. Stammen, J. A., Mechanicalproperties of a novel PVA hydrogel in shear and unconfined compressionBiomaterials, 2001 22: p. 799-806.

As cryogels are crosslinked by physical and not chemical means, there issome concern about their structural stability. The modulus of PVA, inaqueous solution increases with soak time in distilled water at constanttemperature. In one experiment, conducted over 40 days, the modulusincreased by 50%. Putatively, during aqueous aging, the increase instrength, with the concomitant loss of soluble PVA is the result of anincrease in the order of the supramolecular packing of the polymerchains. There are significant implications in this data for thelong-term storage effects of the freeze-thaw gelled PVA.

It is also important to understand the effects of loss of polymer overtime and how that impacts the local host biological environment. Itshould be noted that in this example, the cryogel was only freeze-thawcycled once, although others have shown PVA dissolution followingmultiple freeze-thaw cycles. In general, there is very littleinformation about the stability of PVA cryogel modulus under repeatedload cycling (fatigue).

As might be expected, the swelling of PVA cryogels at any time pointdecreases with increasing number of freeze-thaw cycles, indicating adensification of the PVA gel, most likely due to a higher crosslinkdensity. In the long term, following gelation and under staticconditions, the ultimate swelling ratio decreases while the modulusincreases with time.

In freeze-thaw processing temperature is used to force a phaseseparation of the PVA solutions thus enhancing the gelation mechanism inthe PVA (it should be noted that even at room temperature a solution ofPVA begins to gel weakly over time).

Solvent quality is related to both temperature and the chemicalinteraction of the solvent to the polymer, and is conveniently describedby the Flory interaction parameter χ. In a preferred embodiment, themanipulation of the solvents quality through some process other thantemperature allows much greater control over the gelation process whilepermitting the method to be practiced at approximately room temperature.In particular, by using aqueous based solvents for PVA, the system canbe chosen to minimize impact on materials embedded in the PVA, and canallow fine spatial and temporal control over the final structure of thegel. In a particular solvent, the critical parameter defining thetransition from the first to second solvent (and hence driving the phaseseparation) is known as the theta temperature. An example list ispresented in Table 1 below. TABLE 1 Theta temperatures for PVA invarious solvents (Brandrup, J. & Immergut, E.H., Polymer Handbook, 3Ed.1989, NY, John Wiley & Sons). Solvent Volume Ratio Theta Temperature [°C.] t-Butanol/Water   32:68 25 Ethanol/Water 41.5:58.5 25 Methanol/Water41.7:58.5 25 i-Propanol/Water 39.4:60.6 25 n-Propanol/Water 35.1:64.9 25NaCl/Water 2 Moles/L 25 Water — 97

Physically cross-linked PVA gels may also be produced through thermalcycling (not necessarily with freezing) combined with dehydration. Suchgels are potentially suitable for use in load bearing applications (i.e.artificial articular cartilage). Examination of the material propertiesof this thermally cycled PVA found that the material distributes stressmore homogeneously than stiff single-phase biomaterials (ultrahighmolecular weight polyethylene (UHMWPE)) and preserves the lubricationfilm gap readily in simulated articular cartilage loading. The materialsustained and distributed pressure in the thin film of between 1 and 1.5MPa. In transient load tests, the PVA withstood and distributed loads ofnearly 5 MPa.

Studies have been conducted that further examined the wear properties oftheir thermally cycled, dehydrated PVA under a variety of conditions.The wear rate found in unidirectional pin-on-disk (against alumina)experiments was comparable to that of UHMWPE (although this test isprobably not the most suitable to perform for biological implants).However, in reciprocating tests, the wear rate was up to 18 timeslarger. To improve the wear properties, PVA of higher molecular weightand additionally cross-linked by gamma-radiation (doses over 50 kGy) wastested. Such treatment reduced the wear rate considerably (to about 7times that of UHMWPE). However, in both radiation and thermallycrosslinked PVA the wear rate does not appear adequate for applicationswhere the opposing surface has high hardness. Additionally, irradiationwould adversely affect bioactive materials loaded into the gel.

Methods in accordance with a preferred embodiment include the following:

PVA solutions. To make the 10% solution, 20 grams of PVA (100 kg/mole;99.3+% hydrolyzed; JT Baker) was dissolved in 180 grams of deionizedwater at 90° C. for one to two hours. To make the 20% solution, 30 gramsof PVA was dissolved in 180 grams of deionized water, the solution wasstirred continuously until 60 grams of water evaporated to generate afinal solution of 20% PVA.

PVA gelation. 4-5 ml of PVA solution of 10 or 20 weight percent wereinjected into pre-wetted Slide-A-Lyzer Dialysis cassettes (Pierce,Rockford, Ill.) with a molecular weight cutoff of 3500 Daltons. The 10%PVA solutions were then immersed in NaCl aqueous solutions of 1.5 M, 2.0M 2.5 M or 3 M. The 20% PVA solution was immersed in 3.0M NaCl. Todemonstrate that the gelation effect was not NaCl/aqueoussolvent-dependent, a 10% PVA solution in a dialyzer cassette wasimmersed in a 50/50 methanol/water solution. After 3 days, all of thecassettes containing 10% PVA solution were removed from their respectivesolvents. The gels were then removed from the cassettes and placed in DIwater for at least 5 days to allow initial PVA crystal dissolution thecassette containing the 20% PVA solution was removed after 3 days. ThePVA gel was removed from the cassette and a portion was stored in DIwater. The remaining PVA gel was returned to the 3M NaCl solution. At 6and 12 days, portions of the 20% PVA gel were removed and placed into DIwater for at least five days before further testing.

Quantitative Characterization

To quantify the effect on the structure of the gels of immersionsolution molarity and time immersed, differential scanning calorimetry(DSC), gravimetric swell ratio analysis and dynamic mechanical analysis(DMA) were performed on the samples.

Differential Scanning Calorimetry. DSC thermograms were obtained usingan instrument, for example, a TA Instruments 1000 (TA Instruments, NewCastle, Del.). Selected wet PVA gel samples between 5 and 15 mg wereremoved from deionized water storage after 5 days, blotted dry andcrimped into anodized-aluminum hermetic pans. Scans were performed at 5°C./min from 5° C. to 120° C. The total enthalpy change for the meltingof the gel physical crosslinks was estimated using a linear integrationfrom the departure from baseline (typically near 40° C.) to return tobaseline (typically near 90° C.). Following DSC analysis, the hermeticpans were punctured, weighed and placed in a vacuum oven fordehydration. After two days of dehydration the pans were reweighed todetermine the percent PVA in the original sample.

Gravimetric swell ratio. Samples of PVA gel from each sample wereremoved from deionized water storage after 5 days, blotted with a tissueand dehydrated in a vacuum oven for 2 days. The gravimetric swell ratiowas calculated as the ratio of the mass of water in the gel to the massof PVA in the gel.

Dynamic Mechanical Analysis. To examine the effect of curing solventquality, dynamic mechanical analysis was performed using a Perkin-ElmerTMA 7 (Perkin Elmer, NJ) on the 10% PVA 3 M NaCl and 2 M NaCl samples.To examine the effect of aging in the curing solvent, DMA was alsoperformed on the 20% 3 M NaCl 3 day and 12 day samples. Samples were cutinto rectangles and tested in unconfined compression with a static loadof 250 mN (10% samples) or 1000 mN (20% samples). The storage (and lossmoduli for the 10% samples) were determined for a frequency sweep from 1to 2 Hz at room temperature.

In a preferred embodiment, forcing poly(vinyl alcohol) polymer chains insolution into close proximity (through a spinodal decompositionmechanism) results in the formation of a physical association that isresistant to dissolution. This methodology generates a PVA hydrogelemploys the controlled use of the second solvents having a χ valuesufficient to cause gelation to force the PVA chains to physicallyassociate. It is critical that the solvent quality is controlledcarefully, and in particular for larger components, that the solvent“front” enters the PVA solution in a controlled manner. NaCl/deionizedwater and methanol/deionized water solutions at concentrations in theneighborhood of their “theta” value for PVA were used to force thephysical association and subsequent gelling of the PVA. Gels formed inthis way are termed “thetagels”.

The physical appearance of the hydrogel depends on the molarity of thesolution into which the PVA solution is immersed. FIG. 3 demonstratesthe progression of the gelation of the PVA hydrogel during exposure toNaCl solutions near the “theta” concentration. As exposure timeincreases, the PVA solution becomes stiff and opaque for the solutionsat or above the theta concentration and temperature. For solutionsappreciably below the theta concentration, little or no gelling isapparent. Immersion of the PVA solution into the 50/50 water/methanolsolution also resulted in the generation of a uniform PVA hydrogel.

FIG. 3 shows 10% PVA solution in dialyzer cassettes after 1 day (top)and 3 days (bottom) of immersion in curing solution. From left to right:1.5 M NaCl, 2.0 M NaCl and 3.0 M NaCl. The 1.5 M solution does not gelthe PVA, but the 2.0 M solution and 3.0 M solution do gel the PVA. Notethe progressive opacification of the 2.0 M gel and the shrinkage of the3 M gel from the edges of the cassette as the sample compacts with time(indicated with arrow).

FIG. 4 demonstrates the difference between 10% PVA exposed to 3.0 M and2.0 M solutions for 3 days (after photographed subsequent equilibrationin deionized water). PVA gels were generated by immersion in 3.0 M (leftimage of each pair) and 2.0 M (right image) NaCl immersion solution.Note that the gels are uniform and opaque. The gel exposed to 3.0 M NaClswells less and is more compact following equilibration in deionizedwater. The hydrogels that result are uniform and opaque. The PVA exposedto 2.0 M NaCl is more highly hydrated than that exposed to the 3.0 MNaCl. The increased swelling is an indication that the density ofphysical crosslinks is lower in the gel exposed to the 2 M NaClsolution. Thus, gels formed in this way are “tunable” with respect tomechanical properties. Further, gradient gels can be made using themethod through manipulation of the spatial NaCl concentration.

FIG. 5 shows a hydrogel formed from a 10% PVA solution that was exposedto a spatially varying NaCl concentration. Note, the variation in boththe translucency of the gel and in the swelling ratio. The opaque partof the gel was exposed to 3.0 M NaCl while the clear part was exposed toa concentration below the theta concentration (2.0 M at roomtemperature). The ability to generate a gradient is relevant to thegeneration of a total disk replacement nucleoplasty, with a rigid outerlayer (annulus fibrosis) and a softer center (nucleus pulposus).

Differential Scanning Calorimetry

For thermally cycled PVA gels, an endotherm between 30 degrees Celsiusand 90 degrees Celsius represents the energy required to disrupt thephysical crosslinks formed during the thermal processing. For PVAthetagels in accordance with a preferred embodiment, a similar endothermwas present, FIG. 6 compares the DSC thermogram of a thermally cycledPVA cryogel and a PVA hydrogel formed in 3.0M NaCl. The transitions havesimilar melting endotherms and occur at virtually the same temperature.

The enthalpy change of this endothermic transition gives a goodindication of the amount of crosslinking in the gel as a result of thesolution conditions. For a 10% PVA solution for example the enthalpychange obtained after immersion for 3 days in a 2.0 M solution of NaClwas 16.9 J/g. In contrast the same initial PVA solution yielded anenthalpy change of 19.9 J/g after 3 days in a 3M solution. This resultindicates that solution concentration and soak time both positivelyimpact the amount of physical crosslinking in the gel.

Gravimetric Swell Ratio

In a preferred embodiment, increasing the molarity of NaCl in thesolvent increases the amount of PVA present in the hydrogel per unitmass. FIG. 7 shows the relationship between the percentage of PVA in thegels (fully equilibrated in deionized water) and the molarity of thesolution in which they were cured. Because the PVA is not rigidly heldin the dialysis cassette, it is free to expand or contract under theinfluence of the local forces on the PVA during the gelation process. Itis therefore possible for the final PVA concentration to exceed that ofthe initial PVA solution if the PVA gel has collapsed. FIG. 8 shows thegravimetric swelling ratio for PVA cured in solutions of varying NaClmolarity. For the 20% PVA solution, the 3 day value of swelling ratioand percentage of PVA matched that of the 10% PVA solution (not shown).After 12 days of immersion in 3 M NaCl (and 5 days of equilibration indeionized water), the 20% PVA solution formed a get that was 29% PVA.

Dynamic Mechanical Analysis

In a preferred embodiment, the solution conditions and time of aginghave a marked effect on the visible structure of the gels, and on theirthermal properties. Both effects suggest that there is likely to be aninfluence on the mechanical properties as well. This supposition is bornout by qualitative examination of the samples, but for more rigorousanalysis mechanical testing was performed on the samples using DMA.FIGS. 9 and 10 present data taken from three of the samples. FIG. 9shows that the complex modulus of the sample increases with aging(keeping all other solution conditions constant). In fact, this increasein the modulus is also paralleled by a densification of the final PVAgel. FIG. 10 examines the change in modulus corresponding to the changein solution molarity (once again keeping all other parameters constant).In this FIG. 10, there is a clear indication that the storage (i.e.,elastic component) modulus rises sharply as the solution molarity isincreased (remember at room temperature 2 M NaCl is approximately atheta solvent) whereas the loss (i.e., damping) modulus is barelyaffected.

Cryogels have a fairly low resolution with regard to their finalproperties because each thermal cycle produces a dramatic change in thematerial properties of the gel. The thetagels produced demonstrate thatthe concentration of the solvent produces a monotonic decrease inswelling ratio once the “theta” value is passed (See FIG. 7). Thus, theultimate crosslink density can be fine tuned in proportion to theresolution achievable in the solvent concentration. For example, for the10% PVA solution immersed in NaCl, the weight percentage of PVA in thefinal gel varies at a rate of about 7% per mole NaCl.

In preferred embodiments, the PVA thetagels can be made that exhibit asmooth gradient in spatial properties. Gradient properties cannot easilybe manufactured in cryogels. Instead, the usual approach is to generatean array of stacked lamellae independently that must be joined indissolved PVA and then cycled again. Sharp differences in modulus insuch an array create a material with undesirable mechanical propertiesand with inhomogeneous interfaces. A preferred embodiment includes acomposite annulus fibrosus/nucleus pulposus implant, that benefit fromtechnology enabling a smooth gradient in mechanical properties, whereina central lower modulus “pulposus” provides adequate compressivestrength and a higher modulus peripheral “annulus” minimizes creep andundesirable distortion.

Modulus Enhancement: Incorporation of Ionic Species

For thetagels produced in NaCl, it is possible to include natural(hyaluronic acid) or synthetic (PAA) polymers to create gels with strainvariable compressive moduli. Gelling a PVA/PAA solution in strong NaClwill shield the ionizable charges in the PAA while the PVA iscrosslinked around the collapsed PAA. Re-equilibration in deionizedwater will allow expansion of the PAA and pre-stress the PVA matrix. Theresulting construct has a very different mechanical compressive modulusdue to the repulsion of the fixed charges on the incorporated PAA.

Generating Gradient Thetagel

In another preferred embodiment, to make the gradient thetagel: 10%, 20%and 30% solutions of 100 kg/mole PVA are made as described hereinbefore.A dialyzer cassette is split in half and each half bonded to one side ofa 1×1×1 cm plexiglass box that is filled with the 10% PVA solution. Thesealed box is placed into a temperature controlled “Ussing” stylechamber where it is subjected to a constant 4 molar NaCl concentrationdifference (see FIG. 10). After the number of days where further changesin the gel are insignificant, the gradient gel is removed from thechamber and placed in deionized water for five days prior to furthertesting. Resulting gels are tested as described hereinbefore.

In another embodiment, spatial gradient can be generated using temporaloscillations in concentration. The concentration in the chamber can bemodulated temporally to provide a gel, having a softer interior regionthan the peripheral region where a higher crosslinking occurs.

The chamber 100 includes a cartridge 140 containing a gel 160. Thechamber can be divided into sub-chambers or regions including twoimmersion solvents 112 and 122. In a preferred embodiment, the solventshave the same concentration. In another preferred embodiment, theimmersion solvents have different concentrations that cause a spatialgradient in the gel. Membranes 150, 154 are permeable membranes thatallow the immersion solvents to selectively flow into the vinyl polymersolution. Membrane 130 provides an impermeable barrier to the flow ofany solvent.

Dehydration

Preferred embodiments of the present invention are directed atcontrollably structuring gels. In a particular embodiment, in order topromote smooth dehydration and to homogenize the physical crosslinkingof the PVA thetagel, the gel or solution of PVA may be immersed in aseries of solutions, or in a bath of smoothly changing solvent quality,each with a higher Flory interaction parameter than the previoussolution. This prevents the local “crashing out” of PVA at the surfacedirectly in contact with the immersion solution. The term “crashing out”as used herein is associated with a phenomenon akin to precipitationbecause of the Flory interaction parameter. The polymer chains prefer toassociate with themselves instead of the solvent as the Floryinteraction parameter is above the theta point and thus precipitate orcrash out.

In one preferred embodiment, a thetagel may be created by firstimmersing the contained PVA solution into a solvent which has a Floryinteraction parameter that is higher than the theta point for the PVAsolvent pair. After a period of time the contained PVA is immersed inanother solvent, which has a Flory interaction parameter lower than thetheta point for the PVA solvent pair. The process can continue withimmersion of the contained PVA in solutions having successive decreasesin the Flory interaction parameter until the desired interactionparameter value for the final gel is reached.

A method to form a thetagel in accordance with a preferred embodiment ofthe present invention includes immersing contained 5-20% PVA in DI,followed by immersion for a range of 1 hour to 1 day in 2.0 M NaCl,followed by immersion for a time period ranging between 1 hour to 1 dayin 3.0 M NaCl, followed by immersion for a time period of 1 hour to 1day in 4.0 M NaCl, and followed by immersion for a time period rangingfrom 1 hour to 1 day in 5.0 M NaCl.

In another preferred embodiment, the PVA solution may be subjected to agradually changing solvent quality through a similar range ofelectrolyte concentrations by the gradual addition of a concentratedNaCl solution to a DI water bath such that the change of the saltconcentration is slower, or equal to, the diffusion process into thegel.

A method in accordance with a preferred embodiment includes immersingcontained 5-20% PVA in 1 liter of 1.5 M NaCl, and adding 6 M NaCl at arate of 0.5 ml per minute to raise the electrolyte concentration at arate of 0.0038 M/min and reaching 5 M NaCl after approximately 12 hours.

In another embodiment, the PVA solution may be subjected to one or manyfreeze-thaw cycles to fix the gel into a particular shape. It may thenbe immersed in a series of solutions having successively higher Floryinteraction parameters until the final desired Flory parameter isreached.

A method in accordance with a preferred embodiment includes dissolving5-20% PVA in DI, subjecting the solution to freeze-thaw cycles(approximately 1-8 cycles), and subsequently for a period rangingbetween 1 hour to 1 day, immersing the resultant gel in 2.0 M NaCl. Themethod further includes immersing the PVA gel for a time period of 1hour to 1 day in 3.0 M NaCl, followed by immersion for a time periodranging from 1 hour to 1 day in 4.0 M NaCl and subsequently immersingfor a time period of 1 hour to 1 day in 5.0 M NaCl.

In an alternate preferred embodiment, a method to form a gel includesdissolving a 5-20% PVA in DI, adding NaCl to the PVA solution togenerate a concentration from 0.01 to 2 M NaCl in the PVA solution andthen subjecting the PVA/NaCl solution to between 1 to 8 freeze-thawcycles.

Nanostructuring

Polyvinyl alcohol gel is an extremely biocompatible material that can bemade reasonably stiff without the use of chemical crosslinking orirradiation. However, the material properties of the PVA do not matchthe requirements of materials for use in load bearing applications suchas, for example, artificial articular cartilage or intervertebral disks.A nanostructural enhancement of polymer systems in accordance with apreferred embodiment of the present invention indicates that PVA gels,which are already nearly suitable for use in load bearing orthopedicdevices, may become viable candidates for such applications.

Nanostructuring polyvinyl alcohol theta and hydrogels—particles. Theaddition of particles to polymeric materials can improve the mechanicaland thermal properties of the resulting material when compared toformulations of the neat polymer. Recently, it has been shown that theaddition of nanoparticles to polymers can generate similar enhancementsin the material properties, but with much lower particulateconcentrations than those required of micron sized particles. This isparticularly true when the material properties are dependent on surfacearea. In accordance with preferred embodiments, to strengthen polyvinylalcohol thetagels or hydrogels, the dispersion of uncharged nanoscaleparticles or charged nanoscale particles with uniform or spatiallyvarying surface charges into the solution prior to gelation enhances themechanical and thermal properties of the final gel. Nanoscale particles,if dispersed properly, provide regular nucleation sites for physicalcrosslinking by adsorbing PVA chains to their surfaces in accordancewith a preferred embodiment of the present invention. As in rubbertoughened plastics, these nanoparticles also act as stressconcentrators, thus toughening the gel. Nanoscale particles that mayenhance the properties of PVA gels are, for example, clays (for example,but not limited to, Laponite, montmorillonite), fumed silica, titaniumdioxide or hydroxyapatite. Surface treatments and modifications, such asend grafting of polymers also adjust the way in which the particlesinteract with the polymer gel matrix in accordance with a preferredembodiment of the present invention. These particles may also bebiologically active, such as, for example, capable of releasing drugs topromote growth, or reduce inflammation. Nanostructuring is not limitedto thetagels in accordance with a preferred embodiment of the presentinvention. However, the thetagels in accordance with the presentinvention allow the formation of physical crosslinks around chargedparticles under solution conditions where the debye length is reducedcompared to the working solution. Thus when the gel is replaced in theworking solution of lower electrolyte concentration the particlesinteract through electrostatic forces and add compressive strength toPVA thetagels as compared to PVA freeze-thaw gels.

In one embodiment, nanoparticles are dispersed into solutions of PVA.The solvent may be water, DMSO, methanol or any other solution thatexhibits a Flory interaction parameter that is lower than the thetapoint for the PVA solvent pair during solution preparation. ThePVA/nanoparticle mixture is then subjected to at least one freeze-thawcycle. Subsequent to the freeze-thaw cycling, the gelled PVA is immersedin a solvent that has a Flory interaction parameter near or higher thanthe theta point for the PVA/solvent pair to induce further physicalcrosslinking of the PVA/nanoparticle mixture.

A method in accordance with a preferred embodiment of the presentinvention includes mixing: 5-20% PVA in DI with 1-10% fumed silica,freeze-thawing (1-8 cycles) the solution, followed by immersion for atime period ranging from 1 hour to 5 days in 2-5 M NaCl.

In another embodiment, the PVA/nanoparticle mixture is gelled byimmersion into a solvent that has a Flory interaction parameter near orhigher than the theta point for the PVA/solvent pair to induce physicalcrosslinking of the PVA/nanoparticle mixture. No freeze-thaw cycling isnecessary in this embodiment.

A method in accordance with a preferred embodiment of the presentinvention includes mixing 5-20% PVA in DI with 1-10% fumed silica,followed by immersion for a time period ranging from 1 hour to 5 days in2-5 M NaCl.

In another embodiment, the composite gels resulting from the twoexamples described hereinbefore are subject to further freeze-thawcycles.

In another embodiment, PVA solutions or gels containing nanoparticlesare subject to the dehydration protocol as described hereinbefore. Amethod in accordance with a preferred embodiment of the presentinvention includes mixing 5-20% PVA in DI with 1-10% fumed silica,subjecting the solution for 1-8 cycles of freeze-thawing, followed byimmersion for a time period ranging from 1 hour to 1 day in 2.0 M NaCl,followed by immersion for a time period ranging from 1 hour to 1 day in3.0 M NaCl, followed by immersion for a time period ranging from 1 hourto 1 day in 4.0 M NaCl and subsequently followed by immersion for a timeperiod ranging from 1 hour to 1 day in 5.0 M NaCl.

Nanostructuring polyvinyl alcohol thetagels and cryogels—functionalizedmolecular additives. The addition of particles to the PVA solution priorto gelation can provide enhancement of the thermal and mechanicalproperties of the gel. However, there is a class of molecular additivesthat can be functionalized to promote physical crosslinking and cansimultaneously act as stress concentrators. Polyhedral oligomericsilsesquioxane (POSS) can enhance mechanical properties of polymericmaterials. Since the POSS molecules can be functionalized, they can betuned to associate with the PVA chains to enhance interchaincrosslinking and to act as stress concentrators. Their extremely smallsize and large number of functionalized groups has the potential toprovide better results than nanoparticle seeding.

All of the methods for generating thetagels or cryogels describedhereinbefore may be applied to solutions containing dispersedfunctionalized POSS molecules. In one preferred embodiment, POSSfunctionalized to display negatively charged oxygen groups can be usedto promote hydrogen bonding. The functionalized POSS is dispersed intoaqueous PVA solution and subjected to theta or freeze-thaw gelation(ranges 0.01 mM to 1 M OctaTMA POSS (tetramethyl ammonium salt) and5-20% PVA in solution).

In another preferred embodiment, POSS functionalized to display alcoholgroups is dispersed into PVA and subjected to theta or freeze-thawgelation (ranges 0.01 mM to 1 M Octahydroxypropyldimethylsilyl POSS and5-20% PVA in solution)

In another embodiment, POSS functionalized to display at least one PVAchain and at least one carboxyl or sulfate group can be used to producean extremely hydrophilic, tough artificial cartilage. The preferred POSSconstruct has at least one PVA chain at opposite corners of the POSSwith the 6 remaining functional groups expressing sulfate or carboxylgroups. This structure can be “stitched” into the PVA gel network viathe thetagel process or freeze-thawing to produce an artificialcartilage with tunable properties.

FIG. 12 illustrates a quick freeze deep etch (QFDE) image of PVA gelstructure in accordance with a preferred embodiment of the presentinvention wherein the PVA gel is formed by immersion in 5 M NaCl for 3days. The bar represents 100 nm. QFDE preserves the gel structure in itshydrated state.

FIGS. 13A and 13B are a cross-sectional and a close-up view of thecross-section of a PVA gradient hydrogel, respectively, prepared byfilling Plexiglass tubing with 10% PVA solution, performing one freezethaw cycle (8 hours at −21° C.; 4 hours at room temperature) thenimmersing in 3 M NaCl bath for at least 3 days, and subsequentlydehydrating in air for 60 hours and returning to deionized (DI) water inaccordance with a preferred embodiment of the present invention. FIGS.13A and 13B illustrate the presence of radial gradients in PVA inducedby air dehydration.

FIG. 14 illustrates a cross-sectional view of a PVA gradient hydrogelprepared by filling dialysis cartridge with 10% PVA solution, thenimmersing in a chamber having 3 M NaCl on one side and 6 M NaCl on theother side for 3 days in accordance with a preferred embodiment of thepresent invention. FIG. 15 illustrates a close-up view of thecross-section of the PVA gradient hydrogel of FIG. 14 on the 6 M NaClside prepared by filling the dialysis cartridge with 10% PVA solution,and then immersing in a chamber with 3 M NaCl on one side and 6 M NaClon the other for 3 days. FIGS. 14 and 15 illustrate the presence oflinear gradients in PVA induced by static NaCl solution gradient.

FIGS. 16-19 illustrate nanostructured PVA gels in accordance withpreferred embodiments of the present invention. More particularly, FIG.16 illustrates a nanostructured PVA hydrogel prepared by mixing asolution of 10% PVA and 2 weight percent Laponite clay, subjecting to a1 freeze-thaw cycle, then exposing the solution to a 4 M NaCl solutionfor at least 3 days in accordance with a preferred embodiment of thepresent invention.

FIG. 17 illustrates a nanostructured PVA hydrogel prepared by mixing asolution of 10%, PVA and 4 weight percent of silica, titrating to pH=3,subjecting to a 1 freeze-thaw cycle, then exposing to a 4 M NaClsolution for at least 3 days in accordance with a preferred embodimentof the present invention.

FIG. 18 illustrates a nanostructured PVA hydrogel prepared by mixing asolution of 10% PVA and 4 weight percent silica, titrating to pH=10,subjecting to 1 freeze-thaw cycle, then exposing to 4 M NaCl solutionfor at least 3 days in accordance with a preferred embodiment of thepresent invention.

FIG. 19 illustrates a nanostructured PVA hydrogel prepared by mixing asolution of 10% PVA and 0.001 M octaTMA POSS in water, then subjectingto 1 freeze-thaw cycle in accordance with a preferred embodiment of thepresent invention.

FIG. 20 graphically illustrates storage moduli for hybrid and controlPVA gels in accordance with preferred embodiments of the presentinvention. The graphs are results of DMA testing on 4% silica/PVAnanostructured gel having a pH=10 (sample number 1), 4% silica/PVAnanostructured gel having a pH=3 (sample number 2), 2% Laponite/PVAnanostructured gel (sample number 3), 0.001 M, 10% PVA+octaTMA POSS(sample number 4), and a control gel (sample number 5). All gels weresubjected to 1 freeze-thaw cycle and then immersed in 3 M NaCl for threedays. Prior to DMA testing the samples were equilibrated in DI water forat least 24 hours.

The embodiments of the present invention provide methods or thecontrolled manipulation of the Flory interaction parameter in a solutionof vinyl polymer, in particular polyvinyl alcohol, to yield a workablefluid that gels in a controlled manner. The control of the solventcondition allows control of the gelation rate, which results in a timeperiod in which the PVA solution has only partially gelled, thuspermitting manipulation or working of the precursor gel prior to finalgelation. During this time period the PVA solution is substantiallyfluid and can be injected, pumped, molded, or undergo any othermanipulative processing step. The final properties of the hydrogel,which include, but are not limited to, percentage crystallinity, crystalsize, free volume, and mechanical properties, are influenced by theinitial vinyl polymer concentration, the gellant concentration, (i.e.,the final solvent quality) in the final mixture, the processingtemperature, and the mixing procedure.

One preferred embodiment of the method is shown in FIG. 21A. FIG. 21Aillustrates a flow chart of a method 400 of forming a PVA hydrogelincluding the step 402 of mixing a vinyl polymer and a first solvent;mixing a gellant in the vinyl polymer solution per step 404; andpreventing gelation of the vinyl polymer physical associations byheating the mixture to a temperature above the melting point of thevinyl polymer per step 406. More particularly, this temperature is abovethe melting point of any physical associations formed in the vinylpolymer. In an alternate embodiment, step 406 precedes step 404. Themethod 400 further includes the step 408 of inducing gelation of thevinyl polymer solution; forming a transiently workable viscoelasticsolution per step 410; controlling or modulating the gelation rate ofthe viscoelastic solution per step 412, for example, by modulating atleast one of the temperature, the pressure and the concentrationgradient; and forming the hydrogel per step 414 in accordance with apreferred embodiment of the present invention.

FIG. 21B illustrates a flow chart of methods of forming and providing avinyl polymer hydrogel in accordance with preferred embodiments of thepresent invention. Theses methods are directed at manufacturing vinylpolymer based hydrogels by modulating solvent quality. The methodsinclude the step 424 of dissolving vinyl polymer in water at, forexample, greater than 80° C. at any desired concentration. The next stepincludes the preparation of a gellant as a powder per step 426, or as asolution per step 430. The gellant can naturally be a liquid per step428. The next step 432 includes providing a gellant in sufficientconcentration to be near (above or below) the critical theta conditionof a subsequent mixture when added to the vinyl polymer solution. Themethod then includes the step 434 wherein the gellant and vinyl solutionare kept separately, the step 436 of loading the components into a twoor more chambered device such as a syringe or a pump, and the step 438of injecting the polymer solution into a region of interest, such as acavity in the body through a mixing apparatus. The solution arrives inthe region of interest substantially mixed per step 410.

The method 400 after step 432 can alternatively include the step 442 ofadding the gellant while mixing the vinyl solution, the step 444 whereinthe solution is mixed until it is homogenous and step 446 wherein thevinyl polymer solution is still in a fluid and workable state. Themethod 400 can include the step 448 of loading the polymer solution intoa syringe or pump followed by step 450 of injecting the solution intothe region of interest in the body before the solution has substantiallycrystallized. In the alternative, after step 446, the method 400 caninclude the step 452 of placing the polymer solution in a mold shapedfor a specific use or the step 454 of blow molding the polymer solutionto form a thin hydrogel membrane or per step 456 of loading the polymersolution into a syringe or pump. Step 458 follows by injecting thesolution into the region of interest and per step 460 triggering thegellant to drop solvent quality. Step 462 can follow the processingsteps 440, 450, 452, 454 or 460 alternatively and includes the permanentcrystallization of the gellant that occurs substantially after the abovelisted processing steps.

In several embodiments, the vinyl polymer is highly hydrolyzed polyvinylalcohol of about 50 kg/mol to about 300 kg/mol molecular weight. Thevinyl polymer solution is about 1 weight percent (wt %) to about 50 wt %of polyvinyl alcohol based on the weight of the solution. In preferredembodiments, the vinyl polymer solution is about 10 weight percent toabout 20 weight percent solution of polyvinyl alcohol based on theweight of the solution.

The first solvent is selected from a group of solvents having a low □value that is not sufficient to enable gelation. In several preferredembodiments, the first solvent is selected from the group including, butnot limited to, of deionized water, dimethyl sulfoxide, a C₁ to C₆alcohol and mixtures thereof.

The second solvent, the gellant, is selected from a group of solventshaving the property that raises the □ value of the resultant mixture ofgellant and vinyl solution to χ>0.5 at a specified temperature. Inseveral embodiments, the gellant is selected from the group including,but not limited to, for example, alkali salts, glycosaminoglycans,proteoglycans, oligomeric length hydrocarbons such as polyethyleneglycol, enzyme-cleavable biopolymers, UV-cleavable polymers, chondroitinsulfate, starch, dermatan sulfate, keratan sulfate, hyaluronic acid,heparin, heparin sulfate, biglycan, syndecan, keratocan, decorin,aggrecan, perlecan, fibromodulin, versican, neurocan, brevican, aphototriggerable diplasmalogen liposome, amino acids such as, forexample, serine or glycine, glycerol, sugars or collagen. The gellantcan be added in the form of a solid or as an aqueous solution.

In one preferred embodiment, the gellant is added by being mixed with asolution of the vinyl polymer held at an elevated temperature,preferably above the melting point of the vinyl polymer. The meltingpoint can be suitably determined by differential scanning calorimetry(DSC). With reference to the dashed curve illustrated in FIG. 6, forPVA, the elevated temperature is at least greater than 57 degreesCelsius, preferably greater than 80 degrees Celsius, more preferablygreater than 90 degrees Celsius. In general, with reference to agraphical representation of differential scanning calorimetry resultssuch as FIG. 6, the most preferred elevated temperature is in the linearportion of the curve at temperatures above the melting point.

The resultant mixture begins to undergo a spinodal decomposition as itmixes and cools. The mixture is injected into a body cavity, whereuponit forms a load-bearing gel over a period of time.

In one preferred embodiment the solvent quality of the entire mixture ofPVA, water and secondary or tertiary components has a Flory interactionparameter of 0.25<□<0.8 and preferably in the range of 0.3<□<0.5.

The preferred embodiments of the invention provide an injectablehydrogel that can be used for orthopedic therapies, including nucleuspulposus augmentation or replacement, and augmentation of load bearingsurfaces in an articulated joint such as, for example, knee or hip. Inthe case of knee or hip augmentation, the injectable hydrogel can beused in early interventional therapy for those patients who, although inpain due to partial loss of articular cartilage are not candidates fortotal knee or hip replacement.

The injectable hydrogel can also be used for non-load bearingapplications for replacement, repair, or enhancement of tissue. It canalso be used topically as a protective coating for burns or wounds. Thepreferred embodiments of the invention are especially suited tominimally invasive applications where small access holes in tissue arerequired. The access holes can have diameters of approximately 1-10 mmand can be located for example, without limitation, in the annulusfibrosis, bone tissue, cartilage, or other tissues.

In another preferred embodiment, the gellant is added to a mixingsolution of the vinyl polymer held at an elevated temperature. Theresultant mixture begins to undergo a spinodal decomposition as it mixesand cools. This mixture can then be used to generate a hydrogel deviceto be manufactured using conventional processing means such as injectionor compression molding, blow molding, calendaring, or any other suitableprocessing step. This device can then be implanted as a load bearingdevice, or some other non-load bearing device such as a nerve cuff or aspart of a drug release system. This device can also be used innon-biological applications as protective hydrogel films, or sealingagents.

In another embodiment, the vinyl solution and gellant are not pre-mixed,but are co-injected through a tube via a mixing, chamber having atortuous path that facilitates mixing. The pre-cursor hydrogel can beinjected using a suitable dispenser directly into the target location.FIG. 28A illustrates a flow chart of method 600 of forming a PVAhydrogel including the steps of providing a vinyl polymer and a firstsolvent in a chamber of a first dispenser per step 602; placing agellant in a chamber of a second dispenser per step 604; mixing thevinyl polymer solution and the gellant in a mixing chamber per step 606and heating the mixture to a temperature above the melting point of thevinyl polymer 608. The method then includes dispensing the mixture perstep 610; delivering the mixture into a region of interest, such as acavity, per step 612; and inducing gelation of the mixture per step 614to form a PVA hydrogel in accordance with a preferred embodiment of thepresent invention. In certain embodiments, the step of providing a vinylpolymer and a first solvent includes the step of mixing the vinylpolymer and the first solvent. Similarly, in certain embodiments, thestep of placing the gellant in a chamber of the second dispenser chamberincludes the step of mixing the gellant and a second solvent.

FIG. 28B illustrates a flow chart of a method 620 of forming a PVAhydrogel in accordance with an alternate preferred embodiment. Thismethod 620 is similar to the method 600 illustrated with respect to FIG.28A however, the step of heating the polymer solution in a chamber of afirst dispenser to a temperature above the melting point of physicalassociations in the polymer (step 626) precedes the step of mixing thevinyl polymer solution and the gellant in the mixing chamber (per step628).

The step of inducing gelation as discussed with respect to the flowcharts in FIGS. 28A and 28B includes the modution of temperation, inparticular to lowering the temperature of the mixture of the vinylpolymer solution and the gellant below the crystallization temperature(melting point of the physical association). In alternate preferredembodiments the step of inducing gelation includes the release of activegellants from an inactive gellant complex. The inactive gellant complexincludes, without limitation, for example, enzyme cleavable polymers,heat denaturable polymers, thermal/chemical/photo/triggered liposomes orhydrogels; thermally triggered irreversible crystalline materials suchas starches, and degradable polymers.

FIG. 28C illustrates a flow chart of a method 650 for forming a vinylpolymer hydrogel in accordance with a preferred embodiment of thepresent invention. The method 650 includes mixing a vinyl polymer andfirst solvent per step 652. The method 650 includes the step 654 ofpouring the solution into one barrel of a multi-barrel syringe, pouringa gellant into another barrel of the multi-barrel syringe per step 656,raising the temperature of the barrel above the melting point of thephysical associations of the vinyl polymer per step 658 centrifuge thebarrel per step 660, and injecting through a static mixture such as acannula having a tortuous path the mixture into a region of interest,for example, into a joint space per step 662.

The method 650 includes in an alternate embodiment the step of 666 ofmixing a gellant into a vinyl polymer solution, pouring the solutioninto a single barrel per step 668, raising the temperature of the barrelabove the melting point of the vinyl polymer physical associations perstep 670, centrifuging the barrel per step 672, and injecting thesolution through a cannula or a syringe needle into the region ofinterest per step 674. The steps of the method 650 can thus providematerial such as nucleus pulposus augmentation for a disk system. Thisgel in accordance with the preferred embodiments conforms to the jointspace or any region of interest.

Further, the method 650 includes for the different embodiments thefollowing steps of inserting a closure device for a disk augmentationper step 664 or injecting concentrated gellant into an opening in thejoint space to locally enhance the gelation rate and final mechanicalproperties per step 676 or per step 678 requiring no further procedurepost the injection of the gellant into the region of interest per steps662, 674. The latter steps 664, 676, provide for the augmentation of forexample, the annulus fibrosis in a disk system.

FIGS. 29A-29F schematically illustrate a method 700 for forming anddispensing a vinyl polymer hydrogel in accordance with a preferredembodiment of the present invention. The method, includes per FIG. 29A,a system being supplied in three aseptic containers or two containerswherein the contents of the containers A and B are combined. The asepticcartridges contain PVA, solvent 1 (water) and gellant, respectively, asillustrated in FIG. 29B. The two components are mixed in a sealedcontainer at a temperature above 80° C. The components are thentransferred to a single barreled holder 704 as illustrated in FIG. 29Cand centrifuged whilst maintaining temperature to remove bubbles perFIG. 29D. As illustrated in FIG. 29E, a nozzle or syringe needle isattached to the single barreled holder which is then assembled onto aplunger system that can be mechanically or electrically actuated(ratcheted) to deliver the mix solution. As described herein before, themixed solution flows for a short period of time before becomingunworkable.

FIGS. 30A-30F schematically illustrate a method for forming anddispensing a vinyl polymer hydrogel in accordance with an alternatepreferred embodiment of the present invention. As illustrated in FIG.30A, the hydrogel system is supplied in three aseptic containers or twocontainers wherein the contents of containers A and B are combined. Theaseptic cartridges contain PVA, solvent 1 and gellant, respectively. Asillustrated in FIG. 30B, the PVA solution is mixed and heated and thegellant is also heated in container C. The ingredients are then loadedinto separate chambers of a twin barrel system while maintaining theelevated temperature as shown in FIG. 30C. The barrel system is thencentrifuged and/or vacuum degassed and injected through a static mixernozzle. As shown in FIG. 30E, a nozzle or syringe needle is attached andassembled onto a plunger system that can be mechanically or electricallyactuated (ratcheted) to deliver mix solution. The mixed solution flowsfor a short period of time before becoming unworkable.

FIGS. 31A-31E schematically illustrate an alternative preferred methodfor forming and dispensing a vinyl polymer hydrogel in accordance withan embodiment of the present invention. This embodiment includes ahydrogel system being supplied in a single cartridge. The aseptic doublebarreled cartridge illustrated in FIG. 31A contains PVA and solvent 1 inone and gellant in the other. The cartridge is heated to remelt the PVAin solution as shown in FIG. 31B. The system is then centrifuged and/orvacuum degassed and injected through a static mixer nozzle as shown inFIG. 31C. As illustrated in FIG. 31D, the nozzle or syringe needle isattached and assembled onto a plunger system that can be mechanically orelectrically actuated (ratcheted) to deliver the mixed solution. Themixed solution flows for a short period of time before becomingunworkable.

FIG. 32 schematically illustrates a dispenser 800 for providing a vinylpolymer hydrogel in accordance with a preferred embodiment of thepresent invention. The dispenser includes a first chamber 810; a secondchamber 812; a first chamber piston rod 814; a second chamber piston rod816; a housing 818; a movable lever 820; a fixed handle 822; a mixingchamber 830; a mixing chamber fitting 832; a dispensing tube 834; adispensing tube fitting 836; a dispensing tube opening 838; atemperature controller 850; a connector 852; a mixing chamberheater/cooler 854; a chamber heater/cooler 856; and a dispensed mixture860 in preferred embodiments, vinyl polymer solution and gellant areprovided as premixed sterile solutions, preferably pre-packaged in firstchamber 810 and second chamber 812, respectively. A heater/cooler 854and 856 can include resistive heating, inductive heating, water jacketor Peltier effect heating/cooling elements. The temperature controller850 can be integral with the dispenser 800, or a separate unit,connected by connector 852. The entire dispenser 800 can be sterile,preloaded and intended for a single use.

In another preferred embodiment, drugs can be mixed with either thevinyl polymer solution or the gellant so that the resultant injected gelcontains an encapsulated drug that can release over time.

In yet another preferred embodiment, a small amount of free radicalscavenger is added to the vinyl polymer solution in a concentration ofapproximately 1 to 1000 parts per million. The free radical scavengercan be any common free radical scavenger known to those skilled in theart, but can include Vitamin E and hydroquinones. The purpose of thefree radical scavenger is to minimize the effects of ionizing radiation,either gamma or electron beam (e-beam), which may be used to sterilizethe material prior to use. Radiation can either crosslink or causescissioning in PVA solutions depending on the concentration of thesolution.

In another embodiment, the final mechanical properties of the hydrogelcan be tailored by varying the initial starting concentration of thevinyl polymer solution, and the concentration of the gellant in thefinal mixture.

In another series of embodiments, the generation of conditions conduciveto force the gelation of the PVA entail the internal release of activeingredients or sequestered materials which can comprise any combinationof or, single gellant listed herein before. To change the theta-point ofthe solvent relative to the PVA, or to alter the co-nonsolvency of thesolvent relative to PVA, there exist potential methods based on thesequestration of “active” molecular species. Such mechanisms serve torapidly release the sequestered active molecular species to achievelocal changes in solvency. If enough of the sequestering moieties aredistributed through the system, it is possible to effect a global changein solvency. Such a method would serve to alleviate precipitationproblems associated with mixing the PVA with a particularly activegellant. Suitable sequestration systems are available, includingliposome sequestration, polymer sequestration, crystallinesequestration, gel encapsulation and degradable encapsulation

In a preferred embodiment, liposome sequestration uses lipid vesicles toseparate their contents from the external environment. This system hasbeen used successfully to induce rapid gelation of polysaccharide andprotein hydrogels. Lipid vesicles can be induced to release theircontents by either thermal or phototriggering methods. In preferredembodiments, the gelation of a PVA solution prepared according to thepresent invention can be accelerated following application of a suitabletrigger. A suitable trigger can be the gel/liposome composition heatedto body temperature. In a preferred embodiment of the present invention,an aqueous PVA solution is mixed with a suspension of thermallytriggerable liposomes containing a concentrated NaCl solution or solidNaCl at a temperature below that necessary to induce release of theNaCl. Upon injection into a region of interest such as a body cavity ator near 37° C., the liposomes release the contained NaCl, changing theFlory parameter of the solution and causing gelation of the PVA. Inother embodiments, other suitable gellants can be sequestered in thelipid vesicles to influence the gelation rate of PVA.

In another preferred embodiment, the sequestration system is based onthe increase of colligative activity of a polymer by cleavage of thepolymer into multiple smaller fragments. In some complex polymersystems, degradation produces fragments that are more soluble than theoriginal molecule, for example, collagen. Such an increase in smaller,more active components shifts the solvency of the overall solution toinduce the gelation of PVA. Some examples of suitable polymericsequestration systems (and their enzymatic cleaving complements) arelisted, without limitation, in the table below. The table is notinclusive however the polymer sequestration concepts are intended toinclude all polymers, in particular, biocompatible polymers orbiopolymers, and their particular polymer degradation mechanisms. In afurther embodiment, the two above approaches can be combined, usingtriggered liposomes to sequester the appropriate enzymes to producetriggerable cleavage of the gellant.

In one embodiment, fully formed purified type I collagen fibrils can bemixed with PVA solution at temperatures below the denaturationtemperature of the collagen. The solution can be heated to induce thedenaturation of the collagen fibrils which would release soluble gelatinmolecules into the PVA, changing the relative solubility of the PVA andpotentially inducing PVA gelation. TABLE 2 Specific Cleavage Methods forSelected Gellants Degradation Degradation specifics Polymer mechanism Orenzyme complement Collagen Type I Heat denaturation Temperature 40-90°C. Collagen Type I Enzyme cleavage Collagenase (MMP I) All remainingcollagens Heat denaturation Temperature 40-90° C. All remainingcollagens Enzyme cleavage All MMP complements (MMPs 3-20) HyaluronicAcid Enzyme cleavage Testicular Hyaluronidase Hyaluron lyase ChondroitinSulfate Enzyme cleavage Chondroitinase family Heparin Enzyme cleavageHeparinase I/III All remaining Enzyme cleavage GAG enzymeglycosaminoglycans complements Aggrecan Enzyme cleavage Aggrecanase Allremaining Enzyme cleavage PG enzyme Proteoglycans complements

In another preferred embodiment, the sequestration method entails theconfinement of active moieties in crystals that can be meltedirreversibly, producing a large change in the activity of thecrystalline component. In one referred embodiment the crystallinecomponent is a starch, comprising amylase molecules, amylopectinmolecules or mixtures thereof that are linear and branched multimers ofglucose. Starch particles normally comprise crystalline and amorphousregions. Upon heating in solution, starch particles absorb water readilyand, upon gelatinization, the starch particles become highly osmoticallyactive. When returning to room temperature, starches gelatinize, but notrecrystallize. Thus, PVA can be mixed effectively with crystallinestarch granules to make a solution where the PVA is soluble. However,upon heating above the gelatinization point of the starch and recooling,the PVA would be forced to gel because of the competition for solventwith the gelatinized starch which is more hygroscopic.

In another embodiment, the sequestration method involves the use ofgel-based capsules, which upon a suitable trigger (for example, withoutlimitation, pH, ionic concentration, temperature, radiation) releasetheir encapsulated contents. In a further embodiment, the sequestrationmethod involves the trapping of the active molecules in a degradablematrix. In preferred embodiments, a suitable biodegradable polymer canbe selected from the group including, but not limited to, apoly(L-lactide), a poly(D,L-lactide), a poly(lactide-co-glycolide), apoly(ε-caprolactone), a polyanhydride, a poly(beta-hydroxy butyrate), apoly(ortho ester) and a polyphosphazene.

In general, a suitable gellant in accordance with the preferredembodiments is a solute that is water soluble, has a higher affinity forwater than PVA. In preferred embodiments, a solid gellant or an aqueoussolution of gellant is added to an aqueous PVA solution. Typically PVAsolutions in the range of from about 1 weight % to about 50 weight % PVAare prepared by adding the desired amount of PVA to warm deionized waterwhile mixing. For example, a 20% PVA solution by weight is prepared bydissolving 20 g of PVA (100 kg/mole; 99.3+% hydrolyzed; JT Baker) in 80g of deionized water heated in a water bath to a temperature of greaterthan 90 degrees Celsius with continuous stirring for a minimum of 15minutes using a vortex mixer (VWR BRAND). The PVA solution obtained wassubstantially clear when fully dissolved and melted. The solution wasplaced in a covered container to avoid evaporation, optionally under amineral oil protective layer.

In one preferred embodiment, 1.4 g of 400 molecular weight poly(ethyleneglycol) (PEG400, Sigma Aldrich) was added gradually to 6.0 g of 10% byweight PVA solution was stirred while stirring on a hot plate at 50degrees Celsius and then the jar was shaken. After initially producingan inhomogeneous solution whilst adding the material became homogeneousand rapidly opaque. The final get was (by weight) 8% PVA, 19% PEG 400and 73% water. FIGS. 22A-22D show the product at four time durationsafter the end of mixing; FIG. 22A, zero minutes; FIG. 22B, 15 minutes;FIG. 22C, 2 hours, under a mineral oil protective layer; and FIG. 22D,one day, out of the jar.

In one preferred embodiment, a PVA hydrogel was prepared by adding 35.6g of aqueous 10 wt % PVA solution to 18.7 g of aqueous 5.1 M NaCl whilemixing, and the resulting mixture aggressively shaken. The solution wasbriefly inhomogeneous before becoming smooth and transparent. Over theperiod of 16 hours the solution became gradually more opaque and gelled.The final gel was 7 wt % PVA, 8 wt % NaCl and 85 wt % water, FIGS.23A-23E illustrate the PVA hydrogel prepared, showing the product atfive time durations after pouring into a covered dish and a flexiblebag. FIG. 23A, zero minutes; FIG. 23B, 20 minutes; FIG. 23C, 1 hour;FIG. 23D, 2 hours; and FIG. 23E, 17 hours. Note that the solution wasfluid for long enough that a circular shape was easily formed. Thesolution was also cast in a flexible bag to demonstrate its abilitiesfor space filling applications in deformable environments.

In another embodiment, an aqueous 10 wt % PVA solution was placed in thelarger barrel of a 4:1 ratio epoxy adhesive gun (3M). Poly(ethylene)glycol with a molecular weight of 400 g/mol was placed in the smallerbarrel. The resulting blend was delivered through a 3 inch static mixingnozzle (3M) into a mold held at room temperature. The resulting mix had8 wt % PVA, 20 wt % PEG 400 and 72 wt % water. The resulting mixture wasobserved to gel inhomogeneously on delivery, but with time resulted in ahomogeneous opaque gel.

In a further preferred embodiment, a PVA hydrogel was prepared by addingNaCl to an aqueous 10 wt % PVA solution at about 95 degrees Celsius(FIG. 24A) while mixing to make a final concentration of 2M NaCl. Afterapproximately 15 minutes, the resulting solution was smooth andhomogeneous (FIG. 24B). The PVA solution obtained was poured into twojars, one of which was equilibrated at room temperature (FIG. 24C, 15minutes; FIG. 24E, one hour), the other of which was placed on shavedice (FIG. 24D, 15 minutes; FIG. 24F, one hour). The solution wasinitially cloudy, but homogeneous. After 1 hour, the gel cooled at roomtemperature was slightly cloudy, whereas the ice-cooled gel waspredominantly transparent.

The two samples were then stored at room temperature. The finalresulting gels after one month exhibited significant syneresis andvisually looked identical, but the rapidly cooled material appeared toturn opaque much faster.

FIGS. 25A-25F illustrate the PVA hydrogels of FIGS. 24A-24F afterstorage. FIG. 25A, cooled one hour at room temperature and stored 12hours at room temperature; FIG. 25B, cooled one hour on ice and stored12 hours at room temperature; FIG. 25C, cooled one hour at roomtemperature and stored one month at room temperature; FIG. 25D, cooledone hour on ice and stored one month at room temperature; FIG. 25E, thePVA gel of FIG. 25C, oriented to show water released due to syneresis;and FIG. 25F, the PVA gel of FIG. 25D, oriented to show water releaseddue to syneresis.

In preferred embodiments, the present invention provides a method forearly treatment of joint disease by providing a polymer cushion formedin situ between load-bearing surfaces in the joint. In one preferredembodiment, a PVA cushion is formed in situ within the hip joint bydislocating the head of the femur, filling the exposed cavity within thejoint with a fluid solution of PVA and gellant, replacing the head ofthe femur and allowing the PVA solution to gel in situ. In an example,illustrated in FIGS. 26A-26D, dry NaCl was added at a moderate rate to a20 wt % aqueous PVA solution warmed in a water bath at about 95 degreesCelsius with continuous stirring to make a 2.1 M NaCl solution. After 1minute the resulting solution was smooth and malleable, resemblingtaffy. Resulting solution was removed from the mixer and placed in achilled polyethylene liner from a Total Hip Replacement (THR) system.The matching cobalt chrome ball from the THR joint inserted into theliner socket and allowed to stand for 1 hour at room temperature. Themold was then placed in deionized water for a further 1 hour, whereuponthe ball was removed from the poly(ethylene) liner. At this point athin, homogeneous and substantially blemish-free hemisphere of PVA wasobtained. FIG. 26A shows the mold formed by the chilled polyethyleneliner and the matching ball from a total hip replacement joint. FIG. 26Bshows the mold after filling the chilled polyethylene liner with the PVAsolution and putting the matching ball in place. FIG. 26C shows themolded PVA in the polyethylene liner after one hour in air at roomtemperature followed by one hour in deionized water at room temperature.FIG. 26D shows the molded PVA product removed from the polyethyleneliner.

Since the mechanism of gelation is chemically non-specific, it ispossible to use virtually any solute that has sufficient osmoticactivity to force PVA self-association. In a further preferredembodiment, the co-nonsolvency was exploited by using anaturally-occurring biocompatible material as a gellant to form a PVAgel. Chondroitin sulfate (CS, Now Foods Bloomingdale, Ind.) was used toinduce the gelation of polyvinyl alcohol. Aqueous 10 wt. % PVA solutionwas prepared and stored at 60° C. until use. In a first example, warmchondroitin sulfate solution (˜80° C.) was added to the warm PVAsolution to generate a 5 wt % PVA, 7 wt % CS mixture. Upon cooling toroom temperature, the mixture formed a weak gel over a period of twodays that remained stable (FIG. 27A). In the second example 600 mg CSwas added directly with continuous stirring to 10 ml of 10 wt % PVAsolution at 60 degrees Celsius. Compared to the previous example, themixture formed a much stiffer gel within minutes and remained stable(FIG. 27B). The success of inducing the gelation of PVA using abiocompatible material that is typically present in the joint spacesuggests the possibility of in situ joint repair or augmentation.

In a further example, serine, a common amino acid in the blood stream ofhumans was dissolved in deionized water to produce a 30 wt % aqueoussolution. The PVA solution and serine solution were then combined inequal parts by volume at less than 90 degrees Celsius and mixedthoroughly, resulting in a solution of 10% PVA, 15% serine and 75%water. The solution stayed fluid while at the higher temperature but asit cooled it gradually gelled, producing a gel after about 1 hour.

In a preferred embodiment, the vinyl polymer solution may include,without limitation, mixtures of vinyl polymers such as polyvinyl alcoholand polyvinyl pyrollidone (PVP) or copolymers of PVP, as described inthe European Patent specification EP 1229 873 B1, the entire teachingsof which are incorporated herein by reference.

In a preferred embodiment, the vinyl polymer solution may include anymixture of components that form physical associations throughmanipulation of relative solvent quality.

In a preferred embodiment, the vinyl polymer solution may include a nanoor microstructuring agent which can include nano and microparticulatessuch as clay or silica, charged or uncharged, and/or nanostructuringfunctionalized molecules such as POSS as described herein before. Thesenano or microparticulates provide nucleation sites that accelerate oraugment the gelation process. Preferred embodiments of the presentinvention benefit from this recognition that nucleation sites providedby any particles of the appropriate size in the vinyl polymer solutionaugment gelation to result in a gel of the desired mechanicalproperties.

Repair of Damaged Intervertebral Disks

In preferred embodiments, the method and dispenser of the presentinvention are used in the repair of a damaged intervertebral disk. FIG.33 is a schematic illustration of a midsagittal cross-section 900through two vertebrae, each vertebra having vertebral body 920 andspinous process 922; the two vertebral bodies enclosing anintervertebral disk 910 comprising annulus fibrosis 912, nucleuspulposus 914 and herniation 916; provided for orientation are verticalaxis 930, anterior-posterior axis 932 and arrow 938 indicating posterioraccess path to the herniation 916. FIG. 34 is a schematic illustrationof a transverse section 940 through a damaged intervertebral disk 910,showing spinous process 922 and transverse process 924 of an adjacentvertebra, annulus fibrosis 912, nucleus pulposus 914, the outline of thespinal cord 942 and herniation 916 protruding through defect 918.

Briefly, the damaged intervertebral disk is repaired by respecting theherniated region, injecting a viscoelastic solution of the vinyl polymerhydrogel of the present invention to replace part or substantially allof the nucleolus pulposus material, controlling the rate of gelation ofthe vinyl polymer hydrogel by methods described herein before, andclosing the injection site. The viscoelastic solution of the vinylpolymer hydrogel can be injected through a defect in the annulusfibrosus at the site of the herniation, and/or through another point ofthe annulus fibrosus. The injection site and the defect can be closed bymodifying the local physical properties of the hydrogel by a furtherapplication of gellant in situ, as described herein before.Alternatively, the injection site and the defect can be sealed andreinforced by the use of known medical devices to seal, reinforce orclose the injection site or other defect of the body cavity. Suitablesuch devices are disclosed in published International PatentApplications WO 01/12107 and WO 02/054978, which are hereby incorporatedby reference in their entirety.

FIG. 35 is an illustration of a step in a method for the repair of adamaged intervertebral disk 910 in accordance with a preferredembodiment of the present invention, showing in transverse section 960 aspinous process 922 and a transverse process 924 of an adjacentvertebra, annulus fibrosis 912, nucleus pulposus 914, the outline of thespinal cord 942 and dispensing tube 934 introduced through defect 918,dispensing the viscoelastic mixture of gellant and vinyl polymer 952. Inpreferred embodiments, the local physical properties of the hydrogel areadjusted by an addition of a gellant through the same dispensing tube934 following the dispensing of the desired amount of viscoelasticmixture of gellant and vinyl polymer. The additional gellant can be thesame or different from the gellant initially mixed with the vinylpolymer solution.

In alternative embodiments, a sealer can be used to supplement orinstead of modifying the local physical properties of the hydrogel by anaddition of a gellant. FIG. 36 is a schematic illustration of a step inthe repair in accordance with a preferred embodiment of the presentinvention of a damaged intervertebral disk 910, showing in transversesection 962 a spinous process 922 and a transverse process 924 of anadjacent vertebra, annulus fibrosis 912, nucleus pulposus 914, theoutline of the spinal cord 942, defect 918, mixture of gellant and vinylpolymer 952, sealant 970, fixative 918, a fixative delivery instrument950. The sealant 970 is constructed from a material and is formed insuch a manner as to resist the passage of fluids and other materialsaround the sealant 970 and through the defect 918. The sealant 970 canbe constructed from one or any number of a variety of materialsincluding, but not limited to, polytetrafluoroethylene (PTFE), expandedpolytetrafluoroethylene (e-PTFE), NYLON™, MARLEX™, high densitypolyethylene, and/or collagen. See WO 02/054978, incorporated herein byreference. After placement of the sealant 970, fixative 918, such assutures or soft tissue anchors, are placed using fixative deliveryinstrument 950.

In alternative embodiments, a barrier can be used. FIG. 37 is aschematic illustration of a stop in the repair in accordance with apreferred embodiment of the present invention of a damagedintervertebral disk 910, showing in transverse section 964 a spinousprocess 922 and a transverse process 924 of an adjacent vertebra,annulus fibrosis 912, nucleus pulposus 914, the outline of the spinalcord 942, defect 918, mixture of gellant and vinyl polymer 952, barrier974, fixative 972, and fixation delivery instrument 950. The barrier 974is preferably flexible in nature, and can be constructed from a wovenmaterial such as DACRON™ or NYLON™, a synthetic polyamide, polyester,polyethylene, and may be an expanded material such as e-PTFE.Alternatively, the barrier 974 can be a biologic material such ascollagen. The barrier 974 can be expandable such as, for example, aballoon or a hydrophilic material. See WO 02/054978.

If appropriate the viscoelastic solution of the present invention isintroduced at a site other than the defect. FIG. 38 is a schematicillustration of a step in the repair in accordance with a preferredembodiment of the present invention of a damaged intervertebral disk910, showing in transverse section 968 a spinous process 922 and atransverse process 924 of an adjacent vertebra, annulus fibrosis 912,the outline of the spinal cord 942, defect 918, mixture of gellant andvinyl polymer 952 substantially filling the space previously occupied bythe nucleus pulposus, barrier 976, and dispensing tube 934.

The claims should not be read as limited to the described order orelements unless stated to that effect. Therefore, all embodiments thatcome within the scope and spirit of the following claims and equivalentsthereto are claimed as the invention.

1. A kit for providing vinyl polymer hydrogels to a region of interestcomprising: a container of a vinyl polymer; a container of a firstsolvent; a container of a gellant; and a delivery device.
 2. The kit ofclaim 1 further comprising instructions for use.
 3. The kit of claim 1wherein the delivery device is a dispenser.
 4. The kit of claim 1wherein the dispenser further comprises a first chamber.
 5. The kit ofclaim 1 wherein the dispenser further comprises a second chamber.
 6. Thekit of claim 1 wherein the dispenser further comprises a mixing chamber.7. The kit of claim 1 wherein the dispenser further comprises adispensing tube.
 8. The kit of claim 1 wherein the dispenser furthercomprises at least one of a heater and cooler in communication with thedispensing chamber.
 9. The kit of claim 1 wherein the dispenser furthercomprises at least one of a heater and cooler in communication with themixing chamber.
 10. The kit of 1 further comprising a temperaturecontroller.